Mixtures of compounds comprising at least two double bonds and use thereof

ABSTRACT

A mixture of at least two compounds each having at least two double bonds, said mixture having a WFR from 200 to 600 g/mol of double bond and at least two of said compounds each comprising at least two (meth)acrylic esters as double bond component, WFR being given by:  
           ∑     i   =   1     n     ⁢         α   i     ⨯     MW   i       /     Z   i         =     WFR   ⁢           ⁢   where         
           ∑     i   =   1     n     ⁢     α   i       =   1       
 
     α i  is equal to the molar fraction of compound (i) in said mixture, n is equal to the number of compounds in said mixture and n is ≧2,  
     Z i  is equal to the number of double bonds in said compound (i),  
     MW i  is equal to the molecular weight of said compound (i).

The present invention relates to novel mixtures of at least two compounds each having at least two double bonds, said mixture having a WFR from 200 to 600 g/mol of double bond and at least two of said compounds each comprising at least two (meth)acrylic esters as double bond component, WFR being given by: ${\sum\limits_{i = 1}^{n}{{\alpha_{i} \times {MW}_{i}}/Z_{i}}} = {{WFR}\quad{where}}$ ${\sum\limits_{i = 1}^{n}\alpha_{i}} = 1$

α_(i) is equal to the molar fraction of compound (i) in said mixture, n is equal to the number of compounds (crosslinker components) in said mixture and n is ≧2, Z_(i) is equal to the number of double bonds in said compound (i), MW_(i) is equal to the molecular weight of said compound (i), a simplified process for preparing these esters and the use of reaction mixtures thus obtainable.

Swellable hydrogel-forming addition polymers, known as superabsorbent polymers or SAPs, are known from the prior art. They are networks of flexible hydrophilic addition polymers, which can be both ionic and nonionic in nature. They are capable of absorbing and binding aqueous fluids by forming a hydrogel and therefore are preferentially used for manufacturing tampons, diapers, sanitary napkins, incontinence articles, training pants for children, insoles and other hygiene articles for the absorption of body fluids. Superabsorbents are also used in other fields of technology where fluids, especially water or aqueous solutions, are absorbed. These fields include for example storage, packaging, transportation (packaging material for water-sensitive articles, for example flower transportation, shock protection); food sector (transportation of fish, fresh meat; absorption of water, blood in fresh fish/meat packs); medicine (wound plasters, water-absorbent material for burn dressings or for other weeping wounds), cosmetics (carrier material for pharmaceuticals and medicaments, rheumatic plasters, ultrasound gel, cooling gel, cosmetic thickeners, sunscreen); thickeners for oil/water or water/oil emulsions; textiles (gloves, sportswear, moisture regulation in textiles, insoles); chemical process industry applications (catalyst for organic reactions, immobilization of large functional molecules (enzymes), adhesive for agglomerations, heat storage media, filtration aids, hydrophilic component in polymer laminates, dispersants, liquefiers); building and construction, installation (powder injection molding, clay-based renders, vibration-inhibiting medium, assistants in relation to tunneling in water-rich ground, cable sheathing); water treatment, waste treatment (in particular solidification of aqueous waste), water removal (deicers, reusable sandbags); cleaning; agriculture industry (irrigation, retention of meltwater and dew precipitates, composting additive, protection of forests against fungal and insect infestation, delayed release of active ingredients to plants); fire protection (flying sparks) (covering houses or house walls with SAP gel, since water has a very high heat capacity, ignition can be prevented; spraying of SAP gel in the case of fires such as for example forest fires); coextrusion agent in thermoplastic polymers (hydrophilicization of multilayer films); production of films and thermoplastic moldings capable of absorbing water (for example agricultural films capable of storing rain and dew water); SAP-containing films for keeping fresh fruit and vegetables which can be packed in moist films; the SAP stores water released by the fruit and vegetables without forming condensation droplets and partly reemits the water to the fruit and vegetables, so that neither fouling nor wilting occurs; SAP-polystyrene coextrudates for example for food packs such as meat, fish, poultry, fruit and vegetables); carrier substance in active-ingredient formulations (drugs, crop protection). Within hygiene articles, superabsorbents are generally positioned in an absorbent core which comprises other materials, including fibers (cellulose fibers), which act as a kind of liquid reservoir to intermediately store the spontaneously applied liquid insults and are intended to ensure efficient channelization of the body fluids in the absorbent core toward the superabsorbent.

The current trend in diaper design is toward ever thinner constructions having a reduced cellulose fiber content and an increased hydrogel content. The trend toward ever thinner diaper constructions has substantially changed the performance profile required of the water-swellable hydrophilic polymers over the years. Whereas at the start of the development of highly absorbent hydrogels it was initially solely the very high swellability on which interest focused, it was subsequently determined that the ability of the superabsorbent to transmit and distribute fluid is also of decisive importance. It has been determined that conventional superabsorbents greatly swell at the surface on wetting with liquid, so that transportation of liquid into the particle interior is substantially compromised or completely prevented. This trait of superabsorbents is known as gel blocking. The greater amount of polymer per unit area in the hygiene article must not cause the swollen polymer to form a barrier layer to subsequent fluid. A product having good transportation properties will ensure optimal utilization of the entire hygiene article. This prevents the phenomenon of gel blocking, which in the extreme case will cause the hygiene article to leak. Fluid transmission and distribution is thus of decisive importance with regard to the initial absorption of body fluids.

Good transportation properties are possessed for example by hydrogels having high gel strength in the swollen state. Gels lacking in strength are deformable under an applied pressure, for example pressure due to the bodyweight of the wearer of the hygiene article, and clog the pores in the SAP/cellulose fiber absorber and so prevent continued absorption of fluid. Enhanced gel strength is generally obtained through a higher degree of crosslinking, although this reduces retention performance of the product. An elegant way to enhance gel strength is surface postcrosslinking. In this process, dried superabsorbents having an average crosslink density are subjected to an additional crosslinking step. Surface postcrosslinking increases the crosslink density in the sheath of the superabsorbent particle, whereby the absorbency under load is raised to a higher level. Whereas the absorption capacity decreases in the superabsorbent particle sheath, the core has an improved absorption capacity (compared to the sheath) owing to the presence of mobile polymer chains, so that sheath construction ensures improved fluid transmission without occurrence of the gel blocking effect. It is perfectly desirable for the total capacity of the superabsorbent to be occupied not spontaneously but with time delay. Since the hygiene article is generally repeatedly insulted with urine, the absorption capacity of the superabsorbent should sensibly not be exhausted after the first disposition.

Highly swellable hydrophilic hydrogels are especially polymers of (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable grafting base, crosslinked cellulose or starch ethers, crosslinked carboxymethylcellulose, partially crosslinked polyalkylene oxide or natural products which swell in aqueous fluids, for example guar derivatives. Such hydrogels are used as products which absorb aqueous solutions to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in market gardening.

To improve the performance properties, for example Rewet in the diaper and AUL, highly swellable hydrophilic hydrogels are generally surface or gel postcrosslinked. This postcrosslinking is known per se to one skilled in the art and is preferably effected in aqueous gel phase or as surface postcrosslinking of the ground and classified polymer particles.

WO 90/15830 describes crosslinker mixtures of methylenebisacrylamide with diallyltartaramide.

WO 02132964 describes crosslinker mixtures between acrylic ester glycols and allyl compounds.

DE 19646484 describes crosslinker mixtures between allyl ether acrylic ester glycols and acrylic esters or allylamines.

Allyls are less reactive and frequently lead to tough gels in the polymerization.

WO 93/21237 discloses (meth)acrylates of alkoxylated polyhydric C₂-C₁₀ hydrocarbons that are useful as crosslinkers. The trimethylolpropane crosslinkers used correspond to SR 351, SR 454, SR 502, SR 9035 and SR 415. These crosslinkers have 0, 3, 9, 15 or 20 EO units per TMP. WO 93/21237 says it is advantageous to have 3 times 2-7 EO units per TMP and especially 3 times 4-6 EO units per TMP.

The disadvantage with these compounds is that costly and inconvenient purifying operations are needed for at least partial removal of starting materials and by-products; the crosslinkers used in the reference cited have an acrylic acid content of less than 0.1% by weight.

The production of such higher (meth)acrylic esters by acid-catalyzed esterification of (meth)acrylic acid with the corresponding alcohols in the presence of an inhibitor/inhibitor system and in the presence or absence of a solvent such as benzene, toluene or cyclohexane is common knowledge.

Since the formation of the ester from (meth)acrylic acid and alcohol is known to be based on an equilibrium reaction, it is customary to use one starting material in excess and/or to remove the esterification water formed and/or the target ester from the equilibrium in order that commercial conversions may be obtained.

Therefore, in the production of higher (meth)acrylic esters, it is customary to remove the water of reaction and to use an excess of (meth)acrylic acid.

U.S. Pat. No. 4,187,383 describes an esterification process of (meth)acrylic acid with organic polyols at a reaction temperature of from 20 to 80° C. using an equivalent excess of from 2:1 to 3:1.

The disadvantage of this process is that the low reaction temperature means that the reaction times are up to 35 hours and that excess acid in the reaction mixture is removed by neutralization followed by phase separation.

WO 2001/14438 (Derwent Abstract No. 2001-191644/19) and WO 2001/10920 (Chemical Abstracts 134:163502) describe processes for esterifying (meth)acrylic acid with polyalkylene glycol monoalkyl ethers in a ratio of 3:1-50:1 in the presence of acids and polymerization inhibitors and, after deactivation of the acidic catalyst, copolymerization of the residue of (meth)acrylic ester and (meth)acrylic acid at pH 1.5-3.5, and also the use of said residue as a cement additive.

The disadvantage with these processes is that they are restricted to polyalkylene glycol monoalkyl ethers, that the catalyst has to be deactivated and that such copolymers cannot be used as crosslinkers for hydrogels since they only have one functionality.

It is an object of the present invention to provide further compounds which can be used as free-radical crosslinkers for polymers and especially for superabsorbents and to simplify the process for preparing substances which are useful as free-radical crosslinkers for superabsorbents. There shall further be provided crosslinkers whose hydrolysis stability is high and/or which at the same time produce a very readily divisible gel in the manufacturing process for superabsorbents. There shall also be provided crosslinkers whose properties are readily modifiable in that changes in the manufacturing operation and approval of the products shall incur a minimum of additional costs and of inconvenience.

We have found that this object is achieved by a mixture of at least two compounds each having at least two double bonds, said mixture having a WFR from 200 to 600 g/mol of double bond and at least two of said compounds each comprising at least two (meth)acrylic esters as double bond component, WFR being given by: ${\sum\limits_{i = 1}^{n}{{\alpha_{i} \times {MW}_{i}}/Z_{i}}} = {{WFR}\quad{where}}$ ${\sum\limits_{i = 1}^{n}\alpha_{i}} = 1$

-   -   α_(i) is equal to the molar fraction of compound (i) in said         mixture,     -   n≧2,     -   Z_(i) is equal to the number of double bonds in said compound         (i),     -   MW_(i) is equal to the molecular weight of said compound (i).

WFR, the weight-functionality ratio, corresponds to the double bond functionality of the desired compound or crosslinker mixture. The resulting linear system of equations can be solved by means of the Gauss algorithm for the general case (n components) if it has one or more solutions. Such a mixture is readily optimizable by shifting the relative fractions of mixture components. As a result, the specific crosslinker properties of the mixture can also be varied as desired through simple variation of the fractions of its known and approved components in the production of crosslinked polyacrylates (superabsorbents for example) while, for example, keeping the amount of mixture used the same. Hitherto, use levels were varied for this purpose as well as specific new crosslinkers synthesized and approved at appreciable cost and inconvenience.

The specific case of a 2 component mixture gives rise to the following simple system: α_(i)×MW₁ /Z ₁+α₂×MW₂ Z ₂=WFR and α₂=1−α₁, which ultimately reduces to α₁=(WFR−MW₂/Z₂)/(MW₁/Z₁−MW₂/Z₂).

This system has a unique solution when MW₁/Z₁≠MW₂/Z₂ and when MW₁/Z₁<WFR<MW₂/Z₂ or MW₂/Z₂<WFR<MW₁/Z₁.

Preference is given to above mixtures wherein the lower limit is preferably 210, 220, 230 or 240, the upper limit is preferably 550, 500, 450 or 400, the mixture preferably has a WFR between 240 and 400 g/mol of double bond. It is particularly preferable for the lower limit to be 250, 260, 270, 280 or 290, the upper limit to be 390, 380, 370, 360 or 350, particular preference being given to a WFR which has between 250 and 350 g/mol of double bond. Particular preference is given to a range between 300 and 330/340.

Preference is given to above mixtures wherein n is 2, 3 or 4 and more preferably 2.

Preference is given to the above mixtures wherein the MW/Z ratios of two compounds differ by at least 50, 60, 70, 80 or 90 g/mol of double bond, preferably by at least 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240 g/mol of double bond, more preferably at least 250, 260, 270, 280, 290, 300, 310, 320, 330, 340 g/mol of double bond and especially 350, 360, 270, 380, 390 or 400 g/mol of double bond.

Preference is given to above mixtures wherein one compound has an MW/Z ratio of below 400 g/mol of double bond, preferably below 300 g/mol of double bond, more preferably below 200 g/mol of double bond and especially below 150 g/mol of double bond.

Preference is given to above mixtures wherein one compound has an MW/Z ratio of above 400 g/mol of double bond and below 10 000 g/mol of double bond, preferably of above 600 g/mol of double bond and below 1000 g/mol of double bond.

The number n of individual compounds in the inventive mixture which are present at more than 1% by weight of the mixture is preferably at most 10, for example 9, 8, 7 or 6, more preferably 5 or fewer, or 4 or fewer and most preferably 3 and especially 2.

When the individual components of a mixture form a Gaussian distribution of individual compounds, the above remarks apply preferentially to the midpoint values of the individual components.

Preference is given to above mixtures wherein Z of at least one compound is between 2 and 6, preferably 2, 3 or 4, and especially is within this range for both.

Preference is given to the above mixtures wherein the compounds are esters F_(i) which are available through esterification of polyalcohols A_(i) with (meth)acrylic acid and each polyalcohol A_(i) has Z_(i) hydroxyl functions and especially from 2 to 50 carbon atoms.

The molecular weight of the polyalcohols which can be used is generally, unless otherwise stated, below 5000 g/mol, preferably below 2500 g/mol, more preferably below 1500 g/mol, most preferably below 1000 g/mol and especially below 800 g/mol.

Preferred polyalcohols A are polyols, functionalized polyols, alkoxylated polyols, sugar alcohols, partially alkoxylated sugar alcohols, polyetherols, polyesterols, at least partially alkoxylated polyesterols and at least partially saponified alkoxylated polyesterols.

Examples of polyols are trimethylolbutane, trimethylolpropane, trimethylolethane, neopentylglycol, neopentylglycol hydroxypivalate, pentaerythritol, glycerol, 1,2-ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2-ethyl-1,3-propanediol, 2-methyl-1,3-propanediol, hydroquinone, bisphenol A, bisphenol F, bisphenol B, 2,2-bis(4-hydroxycyclohexyl)propane, 1,1-, 1,2-, 1,3- and 1,4-cyclohexanedimethanol, 1,2-, 1,3- or 1,4-cyclohexanediol, but-2-ene-1,4-diol and but-2-yne-1,4-diol, sugar alcohols having a C4 to C6 chain such as sorbitol for example.

The polyols may bear additional functionalities such as for example ether functions (—O—), carboxyl functions (—COOH) or C₁-C₄-alkyloxycarbonyl functions (ester groups), C₁-C₄-alkyl herein denoting methyl, ethyl, iso-propyl, n-propyl, n-butyl, iso-butyl, sec-butyl or tert-butyl.

Examples of such functionalized polyols are ditrimethylolpropane, dipentaerythritol, dimethylolpropionic acid, dimethylolbutyric acid, trimethylolacetic acid, hydroxypivalic acid and the 2-hydroxyethyl- or C₁-C₄-alkyl esters of these acids mentioned.

Preferred polyols are those of the formula (I):

where

-   -   R¹, R² are independently hydrogen, C₁-C₁₀-alkyl, preferably         Cl-C₄-alkyl, C₁-C₁₀-hydroxyalkyl, preferably         hydroxy-C₁-C₄-alkyl, carboxyl or C₁-C₄-alkyloxy-carbonyl,         preferably hydrogen, hydroxymethyl and C₁-C₄-alkyl and more         preferably hydroxymethyl and C₁-C₄-alkyl.

The alkyl radicals may each be straight chain or branched.

Examples of R¹ and R² are hydrogen, methyl, ethyl, isopropyl, n-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-decyl, hydroxymethyl, carboxyl, methoxycarbonyl, ethoxycarbonyl or n-butoxycarbonyl, preferably hydrogen, hydroxymethyl, methyl and ethyl and more preferably hydroxymethyl, methyl and ethyl.

Particularly preferred polyhydric alcohols of the formula (I) are trimethylolbutane, trimethylolpropane, trimethylolethane, neopentylglycol, pentaerythritol, 2-ethyl-1,3-propanediol, 2-methyl-1,3-propanediol, 1,3-propanediol, 1,4-butanediol, dimethylolpropionic acid, methyl dimethylolpropionate, ethyl dimethylolpropionate, dimethylolbutyric acid, methyl dimethylolbutyrate or ethyl dimethylolbutyrate, preference being given to neopentylglycol, trimethylolpropane, pentaerythritol and dimethylolpropionic acid, even more preference being given to neopentylglycol, trimethylolpropane and pentaerythritol and especially to trimethylolpropane and pentaerythritol.

Examples of sugar alcohols are sorbitol, mannitol, maltitol, lsomalt, diglycerol, threitol, erythritol, adonitol (ribitol), arabitol (lyxitol), xylitol and dulcitol (galactitol).

Examples of polyetherols are polytetrahydrofuran having a molar mass in the range from 162 to 2000, preferably in the range from 162 to 1458, more preferably in the range from 162 to 1098, yet more preferably in the range from 162 to 738 and most preferably in the range from 162 to 378, poly-1,3-propanediol and poly-1,2-propanediol having a molar mass in the range from 134 to 1178, preferably in the range from 134 to 888, more preferably in the range from 134 to 598 and most preferably in the range from 134 to 308, polyethylene glycol having a molar mass in the range from 106 to 898, preferably in the range from 106 to 458, more preferably in the range from 106 to 400 and yet more preferably in the range from 106 to 235 and most preferably diethylene glycol, triethylene glycol and tetraethylene glycol.

Useful polyesterols include for example polyesterols preparable by esterification of polycarboxylic acids, preferably dicarboxylic acids, with the abovementioned polyols.

The starting materials for such polyesterols are known to one skilled in the art. Polycarboxylic acids whose use may be preferable are oxalic acid, maleic acid, fumaric acid, succinic acid, glutaric acid, adipic acid, sebacic acid, dodecanedioic acid, o-phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, azelaic acid, 1,4-cyclohexanedicarboxylic acid or tetrahydrophthalic acid, their isomers and hydrogenation products and also esterifiable derivatives, such as anhydrides or dialkyl esters, for example C₁-C₄-alkyl esters, preferably methyl, ethyl or n-butyl esters.

Useful hydroxyl-containing carboxylic acids or lactones include 4-hydroxybenzoic acid, 6-hydroxy-2-napthoic acid, pivalolactone or □-caprolactone. Useful polyols include the abovementioned polyfunctional alcohols, preferably neopentylglycol, trimethylolpropane, trimethylolethane, pentaerythritol, dimethylolpropionic acid or dimethylolbutyric acid.

Preferred examples of such polyesterols are polyesterols of the formula (IIIa-c),

where

-   -   R¹ and R² are each as defined above, and     -   Y is a straight-chain or branched, optionally substituted         alkylene group of 2 to 20 carbon atoms or an optionally         substituted cycloalkylene or arylene group of 6 to 12 carbon         atoms or a single bond.

Examples of Y are a single bond, methylene, 1,2-ethylene, 1,3-propylene, 1,4-butylene, 1,6-hexylene, 1,7-heptylene, 1,8-octylene, cis-1,2-ethenylene, trans-1,2-ethenylene, 1,2-, 1,3- or 1,4-phenylene, 1,2-cyclohex-1-enylene, 1,2-, 1,3- or 1,4-cyclohexylene, 4-carboxy-1,2-phenylene, 2-carboxy-1,4-phenylene or 1-carboxy-2,4-phenylene.

Preferred Y groups are 1,2-ethylene, 1,4-butylene and 1,2-, 1,3 or 1,4-phenylene.

It will be appreciated that the method of making generally produces mixtures which may additionally comprise lower and higher oligomers.

In a further preferred embodiment, reaction mixtures of at least partially hydrolyzed polyesterols are used as polyalcohols A for producing the ester F.

To this end, the polyesterols described above, for example, are at least partially hydrolyzed with a suitable base and subsequently and optionally after removal of the basic constituents remaining in the reaction mixture esterified with the carboxylic acid B.

Useful bases include for example NaOH, KOH, Ca(OH)₂, milk of lime, Na₂CO₃ or K₂CO₃, for example as solid, solution or suspension, preferably in the form of a 10-50% by weight solution and more preferably in the form of a 20-40% by weight aqueous solution.

The extent to which the ester groups in the polyesterol are hydrolyzed, ie cleaved, is for example at least 10% (based on the ester groups in the starting compound), preferably at least 25%, more preferably at least 50%, even more preferably at least 75% and most preferably at least 90%.

When basic constituents, for example the basic salt of the carboxylic acid, are to be removed from the reaction mixture, this may be effected for example via ion exchangers, for example acidic or strongly acidic ion exchangers.

The reaction mixture is subsequently acidified and esterified with the carboxylic acid B as described.

Polyester (meth)acrylates may be prepared in multiple stages or else in a single stage, as described for example in EP-A 279 303, from (meth)acrylic acid, polycarboxylic acid and polyol.

Useful polyalcohols further include alkoxylated polyols and polyesterols which are obtainable by reaction of a polyol or polyesterol with at least one alkylene oxide.

The present invention further provides reaction mixtures of compounds of the formula VII R⁸—(O(CH(R¹⁰)CH(R¹⁰)O)_(y)—C(═O)R⁹)_(x)   (VII), where

-   -   R⁸ is a polyvalent straight-chain or branched C₂-C₁₀-alkyl         radical,     -   R⁹ is independently in each occurrence a straight-chain or         branched C₂-C₁₀-alkenyl radical,     -   R¹⁰ is independently in each occurrence hydrogen or methyl,     -   x is independently in each occurrence a positive integer of 2 or         greater, and     -   y is independently in each occurrence a number from 3 to 8 for         x=2 and a number from 2 to 7 for x=3 or greater.

The underlying alcohol to be esterified has the formula VIIa R⁸—(O(CH(R¹⁰)CH(R¹⁰)O)_(y)—H)_(x)   (VIIa), where R⁸, R¹⁰, x and y are each as defined above.

The compounds of the formula (VII) are generally C₂-C₁₀ polyhydric alcohols VIIa which have been alkoxylated with between 2 and 8 alkylene oxide units per hydroxyl group and wherein the terminal hydroxyl group of each alkylene oxide chain is esterified with a C₂-C₁₀ unsaturated carboxylic acid or ester. Preferably the starting alcohol is a C₃-C₆ polyhydric alcohol which preferably has from 2 to 4 hydroxyl groups. More preferably the starting alcohol is trimethylolpropane, glycerol, pentaerythritol, 1,3-propanediol, propylene glycol, 1,4-butanediol or butylene glycol. Most preference as starting alcohols is given to trimethylolpropane, glycerol and pentaerythritol.

Useful alkylene oxides include for example ethylene oxide, propylene oxide, iso-butylene oxide, vinyloxirane and/or styrene oxide.

The alkylene oxide chain may preferably be made up of ethylene oxide, propylene oxide and/or butylene oxide units. Such a chain can be made up of one species of an alkylene oxide or of a mixture of alkylene oxides. When a mixture is used, the different alkylene oxide units may be arranged in a random pattern or as a block or blocks of each species. Preferably the alkylene oxide is ethylene oxide, propylene oxide or a mixture thereof, more preferably it is ethylene oxide or propylene oxide and most preferably it is ethylene oxide. It is thus preferable for one R⁹ radical per alkylene oxide unit to be hydrogen while the other is methyl or hydrogen and it is more preferable for both the R⁹ radicals to be hydrogen.

The preferred number of alkylene oxide units in each chain is dependent upon the number of chains.

The esterifying agent is a C₂-C₁₀ straight- or branched-chain ethylenically unsaturated carboxylic acid or ester, preferably a C₂-C₄ and more preferably a C₂-C₃ ethylenically unsaturated carboxylic acid, even more preferably acrylic acid, methacrylic acid or ester and most preferably acrylic acid.

The compounds of the formula VII are frequently present as a mixture of compounds described by this formula and by-products resulting from the preparation process.

Particular preference among these compounds VII is given to those compounds—hereinafter referred to as compounds VIIb—which have been reacted with up to six, more preferably up to four and most preferably four ethylene oxide units per hydroxyl group. These compounds VIIb possess enhanced hydrolytic stability.

Preference is similarly given to compounds VII—hereinafter referred to as compounds VIIc—which

for x=2 have been reacted with more than eight, more preferably more than ten, even more preferably more than twelve and especially not less than 15 or

for x=3 or greater with more than seven, more preferably more than nine, even more preferably more than twelve and most preferably not less than 15 ethylene oxide units per hydroxyl group, since these compounds VIIc generally possess enhanced solubility in water.

Also conceivable are compounds VII where y is 0, 1 or 2 for x=2 and 0 or 1 for x=3.

Of particular advantage are mixtures of the compounds VIIb and VIIc, for example mixtures having a VIIb:VIIc weight ratio in the range from 10:90 to 90:10, preferably in the range from 20:80 to 80:20, more preferably in the range from 30:70 to 70:30 and most preferably in the range from 40:60 to 60:40. However, the mixing ratio is primarily determined by desired end product properties of the polymer.

Preferred examples of such alkoxylated polyols are the alkoxylation products (IIa), (IIb) or (IIc) of polyols of the formula (I),

where

-   -   R¹ and R² are each as defined above,     -   k, l, m and q are independently an integer from 1 to 10,         preferably from 1 to 5, more preferably from 3 to 5 and most         preferably 4, and     -   each X_(i) for i=1 to k, 1 to l, 1 to m and 1 to q can be         independently selected from the group consisting of —CH₂CH₂—O—,         —CH₂—CH(CH₃)—O—, —CH(CH₃)—CH₂—O—, —CH₂—C(CH₃)₂—O—,         —C(CH₃)₂—CH₂—O—, —CH₂—CHVin-O—, —CHVin-CH₂—O—, —CH₂—CHPh-O— and         —CHPh-CH₂—O—, preferably from the group consisting of         —CH₂—CH₂—O—, —CH₂CH(CH₃)—O— and —CH(CH₃—CH₂—O—, and more         preferably —CH₂—CH₂—O—,         where Ph is phenyl and Vin is vinyl.

The compounds in question are preferably from singly to quintuply, more preferably from triply to quintuply and most preferably quadruply ethoxylated, propoxylated or mixedly ethoxylated and propoxylated and especially exclusively ethoxylated neopentylglycol, trimethylolpropane, trimethylolethane or pentaerythritol.

Particular preference among these is given to such polyhydric alcohols of the formula (IIb).

Preference is similarly given to a from singly to 20-tuply, preferably from singly to 10-tuply, more preferably from doubly to 10-tuply, even more preferably from doubly to quintuply, especially from triply to quintuply and specifically from triply to quadruply alkoxylated and preferably ethoxylated, propoxylated or mixedly ethoxylated/propoxylated and more preferably ethoxylated glycerol (here exceptionally reckoned in moles of alkoxy groups per mole of glycerol).

Indicated degrees of alkoxylation are each based on the average degree of alkoxylation.

The number average molecular weight M_(n) of the alkoxylated polyols is preferably not more than 1000 g/mol, more preferably not more than 800 g/mol and most preferably not more than 550 g/mol.

The statements concerning the number average and weight average molecular weights M_(n) and M_(w) are here based on gel permeation chromatography measurements using polystyrene as a standard and tetrahydrofuran as a mobile phase. The method is described in Analytiker Taschenbuch Vol. 4, pages 433 to 442, Berlin 1984.

Examples of alkoxylated sugar alcohols are compounds obtainable from sugar alcohols, for example from the above-recited sugar alcohols, by alkoxylation, for example with the above-recited alkylene oxides, preferably with ethylene oxide and/or propylene oxide and most preferably with ethylene oxide.

Examples thereof are

-   -   the recited tetrols which have been alkoxylated on average with         2-30, preferably 2-20, more preferably 3-10 and especially 3, 4,         5, 6, 7 or 8 alkylene oxide units per mole of sugar alcohol,     -   the recited pentols which have been alkoxylated on average with         3-35, preferably 3-28, more preferably 4-20 and especially 4, 5,         6, 7, 8, 9 or 10 alkylene oxide units per mole of sugar alcohol,     -   higher sugar alcohols which have been alkoxylated on average         with 4-50, preferably 6-40, more preferably 7-30, even more         preferably 8-20 and most preferably 10-15 alkylene oxide units         per mole of sugar alcohol.

Preferred alkoxylated sugar alcohols are alkoxylated sugar alcohols wherein at least one hydroxyl group of the sugar alcohol has not been alkoxylated.

Preferred examples of alkoxylated polyesterols are alkoxylated polyesterols of the formula (IVa-c),

where

-   -   R¹, R² and Y are each as defined above,     -   k, l, m, q, r and s are independently an integer from 1 to 30,         preferably from 1 to 20, more preferably from 1 to 10 and most         preferably from 1 to 5, and     -   each X_(i) for i=1 to k, 1 to l, 1 to m, 1 to q, 1 to r and 1 to         s can be independently selected from the group consisting of         —CH₂—CH₂—O—, —CH₂—CH(CH₃)—O—, —CH(CH₃)—CH₂—O—, —CH₂—C(CH₃)₂—O—,         —C(CH₃)₂—CH₂—O—, —CH₂—CHVin-O—, —CHVin-CH₂—O—, —CH₂—CHPh-O— and         —CHPh-CH₂—O—, preferably from the group consisting of         —CH₂—CH₂—O—, —CH₂—CH(CH₃)—O— and —CH(CH₃)—CH₂—O—, and more         preferably —CH₂—CH₂—O—,         where Ph is phenyl and Vin is vinyl.

The compounds in question are preferably nonalkoxylated or from singly to 10-tuply and more preferably from doubly to quintuply ethoxylated, propoxylated or mixedly ethoxylated and propoxylated esterification products of neopentylglycol, trimethylolpropane, trimethylolethane or pentaerythritol with adipic acid, phthalic acid, terephthalic acid or isophthalic acid.

The reaction of alcohols with an alkylene oxide is known per se to one skilled in the art. Possible procedures may be found in Houben-Weyl, Methoden der Organischen Chemie, 4th edition, 1979, Thieme Verlag Stuttgart, ed. Heinz Kropf, Volume 6/1a, Part 1, pages 373 to 385.

When mixedly alkoxylated alcohols are used, the different alkoxy groups comprised therein may be portioned in a molar ratio to each other of for example 0.05-20:1, preferably 0.1-10:1 and more preferably 0.2-5:1.

The viscosity of the polyalcohols which can be used according to the present invention is not subject to any particular requirements bar that they should be readily pumpable to about 80° C., preferably they should have a viscosity below 1000 mPas, preferably below 800 mPas and most preferably below 500 mPas.

When the polyalcohols used in the esterification have three or more hydroxyl groups, it can be sensible for their use according to the present invention, as radical crosslinkers, for them to be merely partially esterified. In other words, in the case of an n-hydric polyalcohol only at least two of the n hydroxyl groups are esterified with the carboxylic acid B.

For n=3 the degree of esterification is at least 2, for n=4 it is at least 2, preferably at least 2.5 and more preferably at least 3, for n=5 or greater it is at least 2, preferably at least 3 and more preferably at least 4.

In such a case, the target stoichiometric excess of carboxylic acid B is dependent on the target degree of esterification and is thus for example 2/n times the above-indicated molar excesses for a degree of esterification of 2 in the case of an n-fold polyalcohol. It will be appreciated that the esterification can also be discontinued, for example by cooling or dilution, once the desired degree of esterification is reached.

Useful ethylenically unsaturated carboxylic acids B for the present invention include compounds having at least one carboxyl group (—COOH), preferably one, and at least one and preferably one ethylenically unsaturated group.

Useful carboxylic acids for the present invention can be aliphatic, cycloaliphatic or aromatic, preferably aliphatic or cycloaliphatic and most preferably aliphatic, straight-chain or branched and optionally substituted by functional groups.

The carboxylic acids generally have from three to ten carbon atoms, preferably from three to five carbon atoms and more preferably from three to four carbon atoms.

Examples of ethylenically unsaturated carboxylic acids B are acrylic acid, methacrylic acid, ethacrylic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, vinylacetic acid, allylacetic acid and crotonic acid.

Preferred carboxylic acids B are α,β-unsaturated carboxylic acids.

Particular preference is given to methacrylic acid and acrylic acid, herein referred to as (meth)acrylic acid, and acrylic acid is most preferred.

Further preferred esters of polyalcohols are indicated in what follows:

where AO is independently at each instance EO or PO,

-   -   where EO is O—CH2-CH2-,     -   PO is independently at each instance O—CH2-CH(CH3)- or         O—CH(CH3)-CH2-

p1+p2+p3 is an integer between 0 and 5, preferably 0 or 3, 4 or 5,

R1, R2, R3 are independently H or CH3.

The EO and PO units have been incorporated in such a way that polyethers are formed and not peroxides.

Preference is given to the above esters F wherein AO is EO or PO, in particular EO.

Particular preference is given to esters F wherein p1, p2+p3=3 or p1=p2=p3=0.

Preference is also given to above esters F wherein at least one AO is PO and at least one further AO is EO.

We have found that the object is further achieved by esters F of the formula Ib:

where EO is O—CH2-CH2-

-   -   PO is independently at each instance O—CH2-CH(CH3)- or         O—CH(CH3)-CH2-     -   m1+m2+m3+n1+n2+n3 is 3, 4 or 5,     -   m1+m2+m3is 1, 2, 3 or 4,     -   R1, R2, R3 are independently H or CH3.

Or else by esters F of the formula Ic:

where EO is O—CH2-CH2-

-   -   PO is independently at each instance O—CH2-CH(CH3)- or         O—CH(CH3)-CH2-     -   m1+m2+m3+n1+n2+n3 is 3, 4or 5,     -   m1+m2+m3 is 1, 2, 3 or 4,     -   R1, R2, R3 are independently H or CH3.

Preference among the esters above is given to esters F wherein m1+m2+m3+n1+n2+n3 or p1+p2+p3 is equal to 3.

Preference among the esters above is given to esters F wherein m1+m2+m3+n1+n2+n3 or p1+p2+p3 is equal to 5.

Particular preference is given to esters F wherein there are 3 PO units in total.

Very particular preference is given to esters F wherein there is at least one PO at each of the 3 alkoxy chains of the glycerol.

Very particular preference is given to esters F where R1, R2 and R3 are identical and especially when R1, R2 and R3 are H.

Preference is further given to the following esters:

where EO in both formula Id and Ie is O—CH2-CH2-,

PO is independently at each instance O—CH2-CH(CH3)- or O—CH(CH3)-CH2-,

n1+n2+n3 is 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59 or 60,

m1+m2+m3 is 4, 5, 6, 7, 8, 9, 10, 11, 12 or 13,

R1, R2, R3 are independently H or CH3.

The EO and PO units have been incorporated in such a way that polyethers are formed and not peroxides.

Preference is given to esters F as defined above wherein n1, n2, n3 are independently 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20.

Particular preference is given to esters F as defined above wherein n1, n2, n3 are independently 9, 10 or 11.

Particular preference is given to esters F as defined above wherein n1, n2, n3 are independently 15, 16, 17, 18, 19 or 20.

Preference is given to esters F as defined above wherein n1+n2+n3 is equal to 28, 29, 30, 31 or 32.

Preference is given to esters F as defined above wherein n1+n2+n3 is equal to 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59 or 60.

Particular preference is given to esters F as defined above wherein n1+n2+n3 is equal to 30.

Particular preference is given to esters F as defined above wherein n1+n2+n3 is equal to 50.

Especial preference is given to esters F as defined above wherein n1=n2=n3=10.

Especial preference is given to esters F as defined above wherein n1=n2=17 and n3=16.

Preference is also given to esters F as defined above wherein m1, m2, m3 are independently 1, 2, 3, 4 or 5.

Particular preference is given to esters F as defined above wherein m1, m2, m3 are independently 1, 2 or 3.

Particular preference is given to esters F as defined above wherein m1, m2, m3 are independently 2, 3, 4 or 5.

Preference is given to esters F as defined above wherein m1+m2+m3 is equal to 4, 5 or 6.

Preference is given to esters F as defined above wherein m1+m2+m3 is equal to 7, 8, 9, 10, 11, 12 or 13.

Particular preference is given to esters F as defined above wherein m1+m2+m3 is equal to 5.

Particular preference is given to esters F as defined above wherein m1+m2+m3 is equal to 10.

Especial preference is given to esters F as defined above wherein m_(i)=m_(k)=3 and m_(l)=4 where i, k, l are all different and selected from the group consisting of 1, 2, 3.

Especial preference is given to esters F as defined above wherein m_(i)=m_(k)=2 and m_(l)=1 where i, k, l are all different and selected from the group consisting of 1, 2, 3.

Very particular preference is given to esters F wherein R1, R2 and R3 are identical, especially when R1, R2 and R3 are H.

Preference is also given to the following esters:

where EO is O—CH2-CH2-,

PO is independently at each instance O—CH2-CH(CH3)- or O—CH(CH3)-CH2-,

n1, n2, n3 are independently 4, 5 or 6,

n1+n2+n3 is 14, 15 or 16,

m1, m2, m3 are independently 1, 2 or 3,

m1+m2+m3 is 4, 5 or 6,

R1, R2, R3 are independently H or CH3.

The EO and PO units have been incorporated in such a way that polyethers are formed and not peroxides.

Preference is given to esters F as defined above wherein n1+n2+n3 is equal to 15.

Particular preference is given to esters F as defined above wherein n1=n2=n3=5.

Preference is also given to esters F as defined above wherein m1+m2+m3 is equal to 5.

Particular preference is also given to esters F as defined above wherein m1=m2=2 and m3=1.

Very particular preference is given to esters F wherein R1, R2 and R3 are identical, especially when R1, R2 and R3 are H.

Preference is also given to the following esters:

-   -   where AO is independently at each instance —O—CHR3-CHR4- or         —CHR3-CHR4-O— where R3 and R4 are independently H, linear or         branched C1-C8-alkyl,     -   p1 is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,         18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33,         34 or 35,     -   p2 is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,         18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33,         34 or 35,     -   n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,         18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33,         34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49,         50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65,         66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81,         82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97,         98, 99, 100,     -   R1 and R2 are independently H or CH3,     -   wherein there is at least one AO in (AO)p1 and also at least one         AO in (AO)p2 where R3 and R4 are not both H at one and the same         time.

The AO units have been incorporated in such a way that polyethers are formed and not peroxides.

Preference is given to esters F as defined above wherein for all AOs R3 and R4 independently have the same meaning, i.e., for example are accessible by alkoxylation of ethylene glycol or polyethylene glycol with just one alkylene oxide, for example propylene oxide.

Preference is given to esters F as defined above wherein R3 or R4 is H.

Preference is given to esters F as defined above wherein R3 or R4 is independently CH3, CH2CH3, (CH2)2-CH3 or (CH2)7-CH3, preferably CH3.

Preference is also given to esters F as defined above wherein p1 is 1, 2, 3, 4 or 5, more preferably 1, 2 or 3, especially 1 or is a number between 14 and 27.

Preference is also given to esters F as defined above wherein p2 is 1, 2, 3, 4 or 5, more preferably 1, 2 or 3, especially 1 or is a number between 14 and 27.

Preference is given to esters F as defined above wherein n is a number between 2 and 50, preferably between 5 and 30 and especially between 10 and 26.

Preference is given to esters F as defined above wherein the diol component (AO)p1-[-O—CH2-CH2-]n-O-(AO)p2 has an average molecular weight between 300 and 500 and especially around 400.

Preference is given to esters F as defined above wherein the diol component (AO)p1-[-O—CH2-CH2-]n-O-(AO)p2 has an average molecular weight 2000 and 4000 and more preferably between 2500 and 3500.

Preference is given to diol components in ester F as defined above that have a symmetrical construction amongst each other with regard to (AO)p1 and (AO)p2. By symmetrical is meant that p1 and p2 are substantially equal, i.e., the absolute amount of the difference between p1 and p2 is 3 or less, preferably 2 or less, more preferably 1 or less and especially is equal to 0.

Preference is further given to diol components in ester F as defined above that have a structurally similar construction to each other with regard to (AO)p1 and (AO)p2. By structurally similar is meant that the respective (AO)p1 and (AO)p2 components are prepared by simultaneous synthesis on the (poly)ethylene glycol and thus are accessible either both from mixtures of different alkoxides (random (AO)p components) or from sequential synthesis (block (AO)p components). Preference is given in particular to diol components wherein all the AOs have the same meaning and are preferably formed from propylene oxide.

Preference is given to esters F as defined above wherein R1 and R2 are identical and preferably H.

According to the present invention, the esters F of the abovementioned formula having the stated meanings can be used for preparing hydrogel-forming polymers capable of absorbing aqueous fluids and especially as internal crosslinkers. Further preferred internal crosslinkers are di(meth)acrylates, especially diacrylates of polypropylene glycol having 2, 3, 4 or 5 and especially 2 or 3 propylene glycol units.

In the above mixtures, one compound may preferably be represented by one of the following formulae:

-   -   where AO is independently at each instance —O—CHR7-CHR8- or         —CHR7-CHR8-O— where R7 and R8 are independently H, linear or         branched C1-C8-alkyl,     -   R5 and R6 are independently H, linear or branched C1-C8-alkyl,     -   n is 1, 2 or 3     -   p1 is 0, 1 or 2,     -   p2 is 0, 1 or 2,     -   p3 is 0, 1 or 2,     -   p4 is 0, 1 or 2,     -   R1, R2, R3, R4 are independently H or CH3,

Similarly, an ester in the mixture may preferably be represented by one of the following formulae:

-   -   where AO is independently at each instance —O—CHR7-CHR8- or         —CHR7-CHR8-O—, where R7 and R8 are independently H, linear or         branched C1-C8-alkyl,     -   R5 and R6 are independently H, linear or branched C1-C8-alkyl,     -   n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,         18, 19 or20,     -   p1 is 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20,     -   p2 is 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20,     -   p3 is 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20,     -   p4 is 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20,     -   R1, R2, R3, R4 are independently H or CH3.

Particular preference is given to such mixtures wherein AO is independently at each instance EO or PO, where EO is O—CH2-CH2- and PO is independently at each instance O—CH2-CH(CH3)- or O—CH(CH3)-CH2- and R5 and R6 are independently H or CH3. Preference is given in particular to those wherein AO denotes block copolymers from EO and PO and the PO units are in the terminal position and esterified with acrylic acid.

Preference is given especially to mixtures of esters selected from the group represented by the formulae Ig, Ih, Ii or Ij with those selected from the group represented by the formulae Ik, Il, Im or In.

Preference is further given to the following esters for use as one component of the mixture: butanediol diacrylate, butanediol-1EO-diacrylate, butanediol-2EO-diacrylate, butanediol-1EO-1PO-diacrylate, butanediol-2PO-diacrylate, butanediol-3EO-diacrylate, butanediol-2EO-1PO-diacrylate, butanediol-1EO-2PO-diacrylate, butanediol-3PO-diacrylate, trimethylolpropane triacrylate, trimethylolpropane diacrylate, trimethylolpropane-3EO-triacrylate, trimethylolpropane-3EO-diacrylate, trimethylolpropane-2EO-1PO-triacrylate, trimethylolpropane-2EO-1PO-diacrylate, trimethylolpropane-1EO-2PO-triacrylate, trimethylolpropane-1EO-2PO-diacrylate, trimethylolpropane-3PO-triacrylate, trimethylolpropane-3PO-diacrylate, trimethylolpropane-4EO-triacrylate, trimethylolpropane-4EO-diacrylate, trimethylolpropane-1PO-3EO-triacrylate, trimethylolpropane-1PO-3EO-diacrylate, trimethylolpropane-2EO-2PO-triacrylate, trimethylolpropane-2EO-2PO-diacrylate, trimethylolpropane-1EO-2PO-triacrylate, trimethylolpropane-1EO-3PO-diacrylate, trimethylolpropane-4PO-triacrylate, trimethylolpropane-4PO-diacrylate, glycerol triacrylate, glycerol diacrylate, glycerol-3EO-triacrylate, glycerol-3EO-diacrylate, glycerol-2EO-1PO-triacrylate, glycerol-2EO-1PO-diacrylate, glycerol-1EO-2PO-triacrylate, glycerol-1EO-2PO-diacrylate, glycerol-3PO-triacrylate, glycerol-3PO-diacrylate, glycerol-4EO-triacrylate, glycerol-4EO-diacrylate, glycerol-1PO-3EO-triacrylate, glycerol-1PO-3EO-diacrylate, glycerol-2EO-2PO-triacrylate, glycerol-2EO-2PO-diacrylate, glycerol-1EO-2PO-triacrylate, glycerol-1EO-3PO-diacrylate, glycerol-4PO-triacrylate, glycerol-4PO-diacrylate, pentaerythritol triacrylate, pentaerythritol diacrylate, pentaerythritol-3EO-triacrylate, pentaerythritol-3EO-diacrylate, pentaerythritol-2EO-1PO-triacrylate, pentaerythritol-2EO-1PO-diacrylate, pentaerythritol-1EO-2PO-triacrylate, pentaerythritol-1EO-2PO-diacrylate, pentaerythritol-3PO-triacrylate, pentaerythritol-3PO-diacrylate, pentaerythritol-4EO-triacrylate, pentaerythritol-4EO-diacrylate, pentaerythritol-1PO-3EO-triacrylate, pentaerythritol-1PO-3EO-diacrylate, pentaerythritol-2EO-2PO-triacrylate, pentaerythritol-2EO-2PO-diacrylate, pentaerythritol-1EO-2PO-triacrylate, pentaerythritol-1EO-3PO-diacrylate, pentaerythritol-4PO-triacrylate, pentaerythritol-4PO-diacrylate, pentaerythritol tetraacrylate, pentaerythritol-5EO-tetraacrylate, pentaerythritol-3EO-tetraacrylate, pentaerythritol-4EO-1PO-tetraacrylate, pentaerythritol-2EO-1PO-tetraacrylate, pentaerythritol-3EO-2PO-tetraacrylate, pentaerythritol-1EO-2PO-tetraacrylate, pentaerythritol-3PO-2EO-tetraacrylate, pentaerythritol-3PO-tetraacrylate, pentaerythritol-4EO-tetraacrylate, pentaerythritol-1EO-4PO-tetraacrylate, pentaerythritol-5PO-tetraacrylate, pentaerythritol-1PO-3EO-tetraacrylate, pentaerythritol-2EO-2PO-tetraacrylate, pentaerythritol-1EO-3PO-tetraacrylate, pentaerythritol-4PO-tetraacrylate.

Particular preference is given to ester mixtures wherein one component is represented by one of the following compounds: glycerol-3EO-triacrylate (G3EOTA), glycerol-3PO-triacylate (G3POTA), glycerol triacrylate (GTA), trimethyolpropane-3EO-triacrylate (TMP3EOTA), trimethylolpropane-3PO-triacrylate (TMP3POTA) and trimethylolpropane triacrylate (TMPTA). The other component is preferably trimethylpropane first alkoxylated with 5PO and then with 30EO and then fully esterified with acrylic acid (TMP5PO30EOTA), and also the trimethylpropane first alkoxylated with 5PO and then with 30EO and then fully esterified with acrylic acid (TMP30EO5POTA).

Very particular preference is given to the mixtures:

G3EOTA with TMP30EO5POTA, G3POTA with TMP30EO5POTA, GTA with TMP30EO5POTA.

These crosslinkers are particularly preferably combined with those of the formula Im. The further observations apply not only to the preparation of the individual esters from polyalcohols but also to the preparation of the ester mixture from mixtures of polyalcohols. The ester F referred to hereinbelow serves as a representative.

We have found that the object is further achieved by a process for preparing an ester F of a polyalcohol A with (meth)acrylic acid, comprising the steps of

-   -   a) reacting a polyalcohol A with (meth)acrylic acid in the         presence of at least one esterification catalyst C and of at         least one polymerization inhibitor D and optionally also of a         water-azeotroping solvent E to form an ester F,     -   b) optionally removing from the reaction mixture at least some         of the water formed in a), during and/or after a),     -   f) optionally neutralizing said reaction mixture,     -   h) when a solvent E was used, optionally removing this solvent         by distillation, and/or     -   i) stripping with a gas which is inert under the reaction         conditions.

In a preferred embodiment

-   -   the molar excess of (meth)acrylic acid over said polyalcohol A         per hydroxyl group to be esterified in A is at least 1.05:1 and     -   the optionally neutralized (meth)acrylic acid comprised in said         reaction mixture after the last step substantially remains in         said reaction mixture.

(Meth)acrylic acid in the context of the present invention comprehends methacrylic acid, acrylic acid or mixtures of methacrylic acid and acrylic acid. Acrylic acid is preferred.

When the ester F is desired in pure form, it can be purified by known separation processes.

The molar excess of (meth)acrylic acid to A (per hydroxyl group to be esterified in the polyalcohol A) is at least 1.05:1, preferably at least 1.1:1, more preferably at least 1.25:1, even more preferably at least 1.5:1 and especially at least 2.5:1.

In a preferred embodiment, (meth)acrylic acid is used in an excess of for example greater than 10:1, preferably greater than 20:1, more preferably greater than 40:1, even more preferably greater than 100:1, especially greater than 150:1 and specifically greater than 200:1.

The esterification products thus obtainable can be used as radical crosslinkers in hydrogels substantially without further purification, specifically without substantial removal of the excess of (meth)acrylic acid and of the esterification catalyst C.

Unless otherwise mentioned, crosslinking as used herein is to be understood as meaning radical crosslinking (gel crosslinking; internal crosslinking; cross-linking together of linear or lightly crosslinked polymer). This crosslinking can take place via free-radical or cationic polymerization mechanisms or other mechanisms, for example Michael addition, esterification or transesterification mechanisms, but is preferably effected by free-radical polymerization.

Hydrogel-forming polymers capable of absorbing aqueous fluids preferably are capable of absorbing at least their own weight, preferably 10 times their own weight and more preferably 20 times their own weight of distilled water and they are preferably capable of achieving this absorption even under a pressure of 0.7 psi.

Useful polyalcohols A for this invention are compounds which have at least two hydroxyl (—OH) functions, preferably at least three, more preferably from three to ten, even more preferably from three to six and especially three to four.

The polyalcohols may be aliphatic, cycloaliphatic or aromatic, preferably aliphatic or cycloaliphatic and most preferably aliphatic, straight chain or branched and optionally substituted by functional groups.

The number of carbon atoms in the polyalcohols is generally from two to 50 and preferably from three to 40.

The reaction of polyalcohols with an alkylene oxide is known per se to one skilled in the art. Possible ways of conducting the reaction we have found in Houben-Weyl, Methoden der Organischen Chemie, 4th edition, 1979, Thieme Verlag Stuttgart, editor Heinz Kropf, volume 6/1a, part 1, pages 373 to 385.

An example of a way to prepare polyalcohols A is to react the polyol first with EO (to obtain the desired polyethylene polyol) and then with PO.

This can be accomplished for example by placing about 72 g of glycol (an example of a polyol) or the corresponding amount of polyethylene glycol with 0.5 g of KOH, 45% in water as an initial charge in an autoclave and dewatering the initial charge at 80° C. and reduced pressure (about 20 mbar). The appropriate amount of propylene oxide is then added at 145 to 155° C. and allowed to react at this temperature under elevated pressure. The reaction has ended when no further change in pressure is observed. The reaction mixture is then stirred for a further 30 min at 150° C. The appropriate amount of propylene oxide is subsequently added at 120 to 130° C. over a prolonged period and likewise allowed to react. After purging with inert gas and cooling down to 60° C., the catalyst is separated off by addition of sodium pyrophosphate and subsequent filtration.

However, it is also possible to use commercially available alkoxylated glycols, for example di- and tripropylene glycol and triblock polymers of the PO-EO-PO type, for example the Pluronic® RPE polymers, of the type 1720, 1740, 2035, 2510, 2520, 2525 or 3110.

The viscosity of the polyalcohols which can be used according to the present invention is not subject to any particular requirements bar that they should be readily pumpable to about 80° C., preferably they should have a viscosity below 1000 mPas, preferably below 800 mPas and most preferably below 500 mPas.

Useful esterification catalysts C for the present invention are sulfuric acid, aryl or alkyl sulfonic acids or mixtures thereof. Examples of aryl sulfonic acids are benzenesulfonic acid, para-toluenesulfonic acid and dodecylbenzenesulfonic acid, and examples of alkyl sulfonic acids are methanesulfonic acid, ethanesulfonic acid and trifluoromethanesulfonic acid. Similarly, strongly acidic ion exchangers or zeolites are useful as esterification catalysts. Preference is given to sulfuric acid and ion exchangers.

Useful polymerization inhibitors D for the present invention include for example phenols such as alkylphenols, for example, o-, m- or p-cresol (methylphenol), 2-tert-butyl-4-methylphenol, 6-tert-butyl-2,4-dimethylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol, 2-methyl-4-tert-butylphenol, 4-tert-butyl-2,6-dimethylphenol, or 2,2′-methylenebis(6-tert-butyl-4-methylphenol), 4,4′-oxydiphenyl, 3,4-(methylenedioxy)diphenol (sesamol), 3,4-dimethylphenol, hydroquinone, pyrocatechol(1,2-dihydroxybenzene), 2-(1′-methylcyclohex-1′-yl)-4,6-dimethylphenol, 2- or 4-(1′-phenyleth-1′-yl)phenol, 2-tert-butyl-6-methylphenol, 2,4,6-tris-tert-butylphenol, 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 4-tert-butylphenol, nonylphenol [11066-49-2], octylphenol [140-66-9], 2,6-dimethylphenol, bisphenol A, bisphenol F, bisphenol B, bisphenol C, bisphenol S, 3,3′,5,5′-tetrabromo-bisphenol A, 2,6-di-tert-butyl-p-cresol, Koresin® from BASF AG, methyl3,5-di-tert-butyl-4-hydroxybenzoate, 4-tert-butylpyrocatechol, 2-hydroxybenzyl alcohol, 2-methoxy-4-methylphenol, 2,3,6-trimethylphenol, 2,4,5-trimethylphenol, 2,4,6-trimethylphenol, 2-isopropylphenol, 4-isopropylphenol, 6-isopropyl-m-cresol, n-octadecylβ-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxyethyl isocyanurate, 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl)isocyanurate or pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 2,6-di-tert-butyl-4-dimethyl-aminomethylphenol, 6-sec-butyl-2,4-dinitrophenol, Irganox® 565, 1141, 1192, 1222 and 1425 from Ciba Spezialitätenchemie, octadecyl 3-(3′,5′-di-tert-butyl-4′-hydroxy-phenyl)propionate, hexadecyl 3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate, octyl 3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate, 3-thia-1,5-pentanediol bis[(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate], 4,8-dioxa-1,11-undecanediol bis[(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate], 4,8-dioxa-1,11-undecanediol bis[(3′-tert-butyl-4′-hydroxy-5′-methylphenyl)propionate], 1,9-nonanediol bis[(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate], 1,7-heptanediaminebis[3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionamide], 1,1-methanediaminebis[3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionamide], 3(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionic acid hydrazide, 3-(3′,5′-di-methyl-4′-hydroxyphenyl)propionic acid hydrazide, bis(3-tert-butyl-5-ethyl-2-hydroxyphen-1-yl)methane, bis(3,5-di-tert-butyl-4-hydroxyphen-1-yl)methane, bis[3-(1′-methylcyclohex-1′-yl)-5-methyl-2-hydroxyphen-1-yl]methane, bis(3-tert-butyl-2-hydroxy-5-methylphen-1-yl)methane, 1,1-bis(5-tert-butyl-4-hydroxy-2-methylphen-1-yl)ethane, bis(5-tert-butyl-4-hydroxy-2-methylphen-1-yl)sulfide, bis(3-tert-butyl-2-hydroxy-5-methylphen-1-yl)sulfide, 1,1-bis(3,4-dimethyl-2-hydroxyphen-1-yl)-2-methylpropane, 1,1-bis(5-tert-butyl-3-methyl-2-hydroxyphen-1-yl)butane, 1,3,5-tris-[1′-(3″,5″-di-tert-butyl-4″-hydroxyphen-1″-yl)meth-1′-yl]-2,4,6-trimethylbenzene, 1,1,4-tris(5′-tert-butyl-4′-hydroxy-2′-methylphen-1′-yl)butane, aminophenols, for example para-aminophenol, nitrosophenols, for example para-nitrosophenol, p-nitroso-o-cresol, alkoxyphenols, for example 2-methoxyphenol (guajacol, pyrocatechol monomethyl ether), 2-ethoxyphenol, 2-isopropoxyphenol, 4-methoxyphenol (hydroquinone monomethyl ether), mono- or di-tert-butyl-4-methoxyphenol, 3,5-di-tert-butyl-4-hydroxyanisole, 3-hydroxy-4-methoxybenzyl alcohol, 2,5-dimethoxy-4-hydroxybenzyl alcohol (syringa alcohol), 4-hydroxy-3-methoxybenzaldehyde (vanillin), 4-hydroxy-3-ethoxybenzaldehyde (ethylvanillin), 3-hydroxy-4-methoxy-benzaldehyde (isovanillin), 1-(4-hydroxy-3-methoxyphenyl)ethanone (acetovanillone), eugenol, dihydroeugenol, isoeugenol, tocopherols, for example α-, β-, γ-, δ- and ε-tocopherol, tocol, α-tocopherolhydroquinone, and also 2,3-dihydro-2,2-dimethyl-7-hydroxybenzofuran(2,2-dimethyl-7-hydroxycoumaran), quinones and hydroquinones such as hydroquinone or hydroquinone monomethyl ether, 2,5-di-tert-butylhydro-quinone, 2-methyl-p-hydroquinone, 2,3-dimethylhydroquinone, trimethylhydroquinone, 4-methylpyrocatechol, tert-butylhydroquinone, 3-methylpyrocatechol, benzoquinone, 2-methyl-p-hydroquinone, 2,3-dimethylhydroquinone, trimethylhydroquinone, 3-methylpyrocatechol, 4-methylpyrocatechol, tert-butylhydroquinone, 4-ethoxyphenol, 4-butoxyphenol, hydroquinone monobenzyl ether, p-phenoxyphenol, 2-methylhydro-quinone, 2,5-di-tert-butylhydroquinone, tetramethyl-p-benzoquinone, diethyl 1,4-cyclohexanedione-2,5-dicarboxylate, phenyl-p-benzoquinone, 2,5-dimethyl-3-benzyl-p-benzoquinone, 2-isopropyl-5-methyl-p-benzoquinone (thymoquinone), 2,6-diisopropyl-p-benzoquinone, 2,5-dimethyl-3-hydroxy-p-benzoquinone, 2,5-dihydroxy-p-benzoquinone, embelin, tetrahydroxy-p-benzoquinone, 2,5-dimethoxy-1,4-benzoquinone, 2-amino-5-methyl-p-benzoquinone, 2,5-bisphenylamino-1,4-benzo-quinone, 5,8-dihydroxy-1,4-naphthoquinone, 2-anilino-1,4-naphthoquinone, anthraquinone, N,N-dimethylindoaniline, N,N-diphenyl-p-benzoquinonediimine, 1,4-benzoquinone dioxime, coerulignone, 3,3′-di-tert-butyl-5,5′-dimethyldipheno-quinone, p-rosolic acid (aurine), 2,6-di-tert-butyl-4-benzylidenebenzoquinone, 2,5-di-tert-amylhydroquinone, nitroxide free radicals such as 4-hydroxy-2,2,6,6-tetra-methylpiperidinyloxy free radical, 4-oxo-2,2,6,6-tetramethylpiperidinyloxy free radical, 4-acetoxy-2,2,6,6-tetramethylpiperidinyloxy free radical, 2,2,6,6-tetramethyl-piperidinyloxy free radical, 4,4′,4″-tris(2,2,6,6-tetramethylpiperidinyloxy) phosphite, 3-oxo-2,2,5,5-tetramethylpyrrolidinyloxy free radical, 1-oxyl-2,2,6,6-tetramethyl 4-methoxypiperidine, 1-oxyl-2,2,6,6-tetramethyl-4-trimethylsilyloxypiperidine, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl 2-ethylhexanoate, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl benzoate, 1-oxyl-2,2,6,6-tetra-methylpiperidin-4-yl(4-tert-butyl)benzoate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)succinate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipate, bis(1-oxyl-2,2,6,6-tetra-methyl-4-piperidinyl)1,10-decanedioate, bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)n-butylmalonate, bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)phthalate, bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)isophthalate, bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)terephthalate, bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)hexahydro-terephthalate, N,N′-bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)adipamide, N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)caprolactam, N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)dodecylsuccinimide, 2,4,6-tris[N-butyl-N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl]triazine, N,N′-bis(1-oxyl-2,2,6,6-tetramethyl-piperidinyl)-N,N′-bisformyl-1,6-diaminohexane, 4,4′-ethylenebis(1-oxyl-2,2,6,6-tetramethyl-3-piperazinone), aromatic amines such as phenylenediamines, N,N-diphenylamine, N-nitrosodiphenylamine, nitrosodiethylaniline, N,N′-dialkyl-para-phenylenediamine, wherein the alkyl radicals can be the same or different and may each independently contain from 1 to 4 carbon atoms and be straight-chain or branched, for example N,N′-di-isobutyl-p-phenylenediamine, N,N′-di-iso-propyl-p-phenylenediamine, Irganox 5057 from Ciba Spezialitätenchemie, N,N′-di-iso-butyl-p-phenylenediamine, N,N′-di-iso-propyl-p-phenylenediamine, p-phenylenediamine, N-phenyl-p-phenylenediamine, N,N′-diphenyl-p-phenylenediamine, N-isopropyl-N-phenyl-p-phenylenediamine, N,N′-di-sec-butyl-p-phenylenediamine (Kerobit® BPD from BASF AG), N-phenyl-N′-isopropyl-p-phenylenediamine (Vulkanox® 4010 from Bayer AG), N-(1,3-dimethyl-butyl)-N′-phenyl-p-phenylenediamine, N-phenyl-2-naphthylamine, iminodibenzyl, N,N′-diphenylbenzidine, N-phenyltetraaniline, acridone, 3-hydroxydiphenylamine, 4-hydroxydiphenylamine, hydroxylamines such as N,N-diethylhydroxylamine, urea derivatives such as urea or thiourea, phosphorus compounds, such as triphenylphosphine, triphenyl phosphite, hypophosphorous acid or triethyl phosphate, sulfur compounds such as diphenyl sulfide, phenothiazine or metal salts, for example copper chloride, copper dithiocarbamate, copper sulfate, copper salicylate, copper acetate, manganese chloride, manganese dithiocarbamate, manganese sulfate, manganese salicylate, manganese acetate, cerium chloride, cerium dithiocarbamate, cerium sulfate, cerium salicylate, cerium acetate, nickel chloride, nickel dithiocarbamate, nickel sulfate, nickel salicylate, nickel acetate, chromium chloride, chromium dithiocarbamate, chromium sulfate, chromium salicylate, chromium acetate or mixtures thereof. Preference is given to the phenols and quinones mentioned, particular preference is given to hydroquinone, hydroquinone monomethyl ether, 2-tert-butyl-4-methylphenol, 6-tert-butyl-2,4-dimethylphenol, 2,6-di-tert-butyl-4-methylphenol, 2,4-di-tert-butylphenol, triphenyl phosphite, hypophosphorous acid, CuCl₂ and guajacol, and very particular preference is given to hydroquinone and hydroquinone monomethyl ether.

Particular preference is given to hydroquinone monomethyl ether, hydroquinone and alkylphenols, optionally in combination with triphenyl phosphite and/or hypophosphorous acid.

Very particular preference is given to α-tocopherol (vitamin E), β-tocopherol, γ-tocopherol or δ-tocopherol, optionally in combination with triphenyl phospite and/or hypophosphorous acid

Very particular preference is given to using just sterically hindered phenols as stabilizers that produce less dark product mixes for the same end-product acid number during the esterification reaction compared with hydroquinone monomethyl ether. An example of such most preferred stabilizers is α-tocopherol.

Stabilization may be further augmented by the presence of an oxygen-containing gas, preferably air or a mixture of air and nitrogen (lean air).

Among the recited stabilizers, preference is given to those which are aerobic, ie those which require the presence of oxygen to fully develop their inhibiting effect.

Useful solvents E for the present invention are particularly solvents which are suitable for azeotropic removal of the water of reaction, if desired, in particular aliphatic, cycloaliphatic and aromatic hydrocarbons or mixtures thereof.

Preference is given to n-pentane, n-hexane, n-heptane, cyclohexane, methylcyclohexane, benzene, toluene or xylene. Particular preference is given to cyclohexane, methylcyclohexane and toluene.

The esterification may be carried out by conventional preparation and/or workup processes for polyhydric alcohols, for example the processes mentioned at the beginning or the processes described in DE-A 199 41 136, DE-A 38 43 843, DE-A 38 43 854, DE-A 199 37 911, DE-A 199 29 258, EP-A 331 845, EP 554 651 or U.S. Pat. No. 4,187,383.

In general, the esterification may be carried out as follows:

The esterification apparatus comprises a stirred reactor, preferably a reactor with circulatory evaporator and an added distillation unit with condenser and phase separation vessel.

The reactor may be for example a reactor with jacketed heating and/or internal heating coils. Preference is given to using a reactor having an external heat exchanger and natural or forced circulation, ie through use of a pump, more preferably natural circulation where circulation is accomplished without mechanical aids.

It will be appreciated that the reaction can also be carried out in a plurality of reaction zones, for example a reactor battery of two to four and preferably two or three reactors.

Suitable circulatory evaporators are known to one skilled in the art and are described for example in R. Billet, Verdampfertechnik, HTB-Verlag, Bibliographisches Institut Mannheim, 1965, 53. Examples of circulatory evaporators are tube-bundle heat exchangers, plate-type heat exchangers, etc.

It will be appreciated that the circulatory system may also include a plurality of heat exchangers.

The distillation unit is of conventional design. It may be a simple distillation unit which if appropriate is equipped with a splash guard or it may be a rectification column. Suitable column internals include in principle all common internals, for example trays, structured packings and/or dumped packings. Preferred trays include bubble trays, sieve trays, valve trays, Thormann trays and/or dual-flow trays, while preferred dumped packings are those of rings, coils, saddles or braids.

In general, from 5 to 20 theoretical plates are sufficient.

The condenser and the separation vessel are of traditional design.

The (meth)acrylic acid and the alkoxylated glycol are generally used in the esterification a) in a molar excess as indicated above. The excess used can be up to about 3000:1, if desired.

Useful esterification catalysts C include those recited above.

They are generally used in an amount of 0.1-5% by weight, based on the esterification mixture, preferably 0.5-5%, more preferably 1-4% and most preferably 2-4% by weight.

If necessary, the esterification catalyst can be removed from the reaction mixture with the aid of an ion exchanger. The ion exchanger can be added directly to the reaction mixture and then subsequently filtered off, or the reaction mixture can be passed through an ion exchanger bed.

Preferably, the esterification catalyst is left in the reaction mixture. However, where the catalyst is an ion exchanger, the ion exchanger is preferably removed, for example by filtration.

Stabilization may be further supported by the presence of an oxygen-containing gas, preferably air or a mixture of air and nitrogen (lean air).

This oxygen-containing gas is preferably metered into the bottom region of a column and/or into a circulatory evaporator and/or passed through and/or over the reaction mixture.

The polymerization inhibitor (mixture) D (as indicated above) is used in a total amount of 0.01-1% by weight, based on the esterification mixture, preferably 0.02-0.8% and more preferably 0.05-0.5% by weight.

The polymerization inhibitor (mixture) D may be used for example as an aqueous solution or as a solution in a reactant or product.

b) The water of reaction formed in the course of the reaction can be distilled off during or after the esterification a), in which case this operation can be augmented by a solvent which forms an azeotrope with water.

Useful solvents E for azeotropic removal of the water of reaction, if desired, include the compounds recited above.

The esterification is preferably carried out in the presence of a solvent.

The amount of solvent used is 10-200% by weight, preferably 20-100% by weight and more preferably from 30% to 100% by weight, based on the sum total of alkoxylated glycol and (meth)acrylic acid.

However, an operation without entrainer is also conceivable, as described for example in DE-A1 38 43 854, column 2 line 18 to column 4 line 45, but in contradistinction to the cited reference with the abovementioned stabilizers.

When the water in the reaction mixture is not removed via an azeotrope-forming solvent, it may be removed by stripping with an inert gas, preferably an oxygen-containing gas and more preferably air or lean air as described for example in DE-A 38 43 843.

The reaction temperature for the esterification a) is generally in the range from 40 to 160° C., preferably in the range from 60 to 140° C. and more preferably in the range from 80 to 120° C. The temperature may remain constant or rise in the course of the reaction and preferably it is raised in the course of the reaction. In this case, the final temperature of the esterification is 5-30° C. higher than the initial temperature. The temperature of the esterification can be determined and controlled by varying the solvent concentration in the reaction mixture, as described in DE-A 199 41 136 and the German application under file reference 100 63 175.4.

When a solvent is used, it can be distilled out of the reaction mixture through the distillation unit added on top of the reactor.

The distillate may selectively be removed or, after condensation, fed into a phase separation apparatus. The aqueous phase thus obtained is generally removed from the system, while the organic phase can be fed as reflux into the distillation unit and/or passed directly into the reaction zone and/or fed into a circulatory evaporator as described in the German patent application under file reference 100 63 175.4.

When used as reflux, the organic phase can be used as described in DE-A 199 41 136 for controlling the temperature in the esterification.

The esterification a) can be carried out with no pressure, at superatmospheric or reduced pressure and is preferably carried out at atmospheric pressure.

The reaction time is generally in the range from 2 to 20 hours, preferably in the range from 4 to 15 hours and more preferably in the range from 7 to 12 hours.

The order in which the individual reaction components are added is not essential to the present invention. All components can be introduced as a mixed initial charge and subsequently heated, or one or more components may be omitted from or only partly included in the initial charge and added only after the initial charge has been heated up.

The (meth)acrylic acid which can be used is not restricted in its composition and may comprise for example the following components: (Meth)acrylic acid 90-99.9% by weight Acetic acid 0.05-3% by weight Propionic acid 0.01-1% by weight Diacrylic acid 0.01-5% by weight Water 0.05-5% by weight Carbonylics 0.01-0.3% by weight Inhibitors 0.01-0.1% by weight Maleic acid or anhydride 0.001-0.5% by weight

The crude (meth)acrylic acid used is generally stabilized with 200-600 ppm of phenothiazine or other stabilizers in amounts which permit comparable stabilization. Carbonylics here refers for example to acetone and lower aldehydes, for example formaldehyde, acetaldehyde, crotonaldehyde, acrolein, 2-furfural, 3-furfural and benzaldehyde.

Crude (meth)acrylic acid here refers to the (meth)acrylic acid mixture which is obtained after absorption of the reaction gases of the propane/propene/acrolein or isobutane/isobutene/methacrolein oxidation in an absorbent and subsequent removal of the absorbent, or which is obtained by fractional condensation of the reaction gases.

It is obviously also possible to use pure (meth)acrylic acid, for example of the following purity: (Meth)acrylic acid 99.7-99.99% by weight Acetic acid 50-1000 weight ppm Propionic acid 10-500 weight ppm Diacrylic acid 10-500 weight ppm Water 50-1000 weight ppm Carbonylics 1-500 weight ppm Inhibitors 1-300 weight ppm Maleic acid or anhydride 1-200 weight ppm

The pure (meth)acrylic acid used is generally stabilized with 100-300 ppm of hydroquinone monomethyl ether or other storage stabilizers in amounts which permit comparable stabilization. For example, the pure (meth)acrylic acid may also be stabilized with vitamin E or other sterically hindered phenols.

Pure or prepurified (meth)acrylic acid generally refers to (meth)acrylic acid whose purity is at least 99.5% by weight and which is substantially free of aldehydic, other carbonylic and high-boiling components.

The aqueous phase, distilled off during the esterification, of the condensate removed via the added column (if present) may generally comprise 0.1-10% by weight of (meth)acrylic acid, and is separated off and removed from the system. The (meth)acrylic acid it comprises may preferably be extracted with an extractant, preferably with any solvent used in the esterification, for example with cyclohexane, at from 10 to 40° C. and a ratio of 1:5-30 and preferably 1:10-20 for aqueous phase to extractant, and returned into the esterification.

Circulation may be further supported by passing an inert gas, preferably an oxygen-containing gas, more preferably air or a mixture of air and nitrogen (lean air) into the circulation or through or over the reaction mixture, for example at rates of 0.1-1, preferably 0.2-0.8 and more preferably 0.3-0.7 m³/m³h, based on the volume of the reaction mixture.

The course of the esterification a) can be monitored by monitoring the amount of water carried out and/or the decrease in the carboxylic acid concentration in the reactor.

The reaction can be ended for example as soon as 90%, preferably at least 95% and more preferably at least 98% of the theoretically expected amount of water has been carried out by the solvent.

The end of the reaction can be detected for example from the fact that substantially no further water of reaction is removed via the entrainer. When (meth)acrylic acid is carried out together with the water of reaction, its fraction is determinable for example by backtitrating an aliquot of the aqueous phase.

The removal of the water of reaction can be dispensed with for example when the (meth)acrylic acid is used in a high stoichiometric excess, for example of at least 3:1, preferably at least 5:1 and most preferably at least 10:1. In this case, a substantial portion of the amount of water formed will remain in the reaction mixture. Merely that fraction of water is removed from the reaction mixture during or after the reaction which is determined by the volatility at the employed temperature and beyond that no measures are carried out to remove the resulting water of reaction. For instance, at least 10% by weight of the resulting water of reaction can remain in the reaction mixture, preferably at least 20% by weight, more preferably at least 30% by weight, even more preferably at least 40% by weight and most preferably at least 50% by weight.

c) After the end of the esterification the reaction mixture can be conventionally cooled to 10-30° C. and optionally by addition of a solvent which may be the same as the solvent optionally used for azeotropic removal of water or a different solvent adjusted to any desired target ester concentration.

In a further embodiment, the reaction can be stopped with a suitable diluent G and diluted to a concentration of for example 10-90% by weight, preferably 20-80%, more preferably 20-60%, even more preferably 30-50% and most preferably about 40%, for example in order to reduce the viscosity.

What is important is that a substantially homogeneous solution forms after dilution.

This is preferably accomplished only relatively shortly before use in-the production of the hydrogel, for example not more than 24 hours before, preferably not more than 20 hours before, more preferably not more than 12 hours before, even more preferably not more than 6 hours before and most preferably not more than 3 hours before.

The diluent G is selected from the group consisting of water, a mixture of water with one or more organic solvents which are soluble in water in any proportion and a mixture of water with one or more monohydric or polyhydric alcohols, for example methanol and glycerol. The alcohols preferably bear 1, 2 or 3 hydroxyl groups and preferably have from 1 to 10 and especially up to 4 carbon atoms. Preference is given to primary and secondary alcohols.

Preferred alcohols are methanol, ethanol, isopropanol, ethylene glycol, glycerol, 1,2-propanediol and 1,3-propanediol.

d) If necessary, the reaction mixture may be decolorized, for example by treatment with active carbon or metal oxides, for example alumina, silica, magnesium oxide, zirconium oxide, boron oxide or mixtures thereof, in amounts for example of 0.1-50% by weight, preferably from 0.5% to 25% by weight, more preferably 1-10% by weight at temperatures of for example from 10 to 100° C., preferably from 20 to 80° C. and more preferably from 30 to 60° C.

This can be effected by adding the pulverulent or granular decolorizing agent to the reaction mixture and subsequent filtration or by passing the reaction mixture through a bed of the decolorizing agent in the form of any desired suitable moldings.

The decolorizing of the reaction mixture can be effected at any desired stage in the workup process, for example at the stage of the crude reaction mixture or after any prewash, neutralization, wash or solvent removal.

The reaction mixture can further be subjected to a prewash e) and/or a neutralization f) and/or an afterwash g), preferably merely to a neutralization f). If desired, a neutralization f) and a prewash e) can be interchanged in the sequence.

(Meth)acrylic acid, and/or catalyst C can be at least partly recovered from the aqueous phase of the washes e) and g) and/or neutralization f) by acidification and extraction with a solvent and reused.

For a pre- or afterwash e) or g), the reaction mixture is treated in a wash apparatus with a wash liquor, for example water or a 5-30% by weight, preferably 5-20% and more preferably 5-15% by weight sodium chloride, potassium chloride, ammonium chloride, sodium sulfate or ammonium sulfate solution, preferably water or sodium chloride solution.

The ratio of reaction mixture to wash liquor is generally in the range from 1:0.1 to 1:1, preferably in the range from 1:0.2 to 1:0.8 and more preferably in the range from 1:0.3 to 1:0.7.

The wash or neutralization can be carried out for example in a stirred container or in other conventional apparatuses, for example in a column or a mixer-settler apparatus.

In terms of process engineering, any wash or neutralization in the process according to the present invention can be carried out using conventional extraction and washing processes and apparatuses, for example those described in Ullmann's Encyclopedia of Industrial Chemistry, 6th ed, 1999 Electronic Release, Chapter Liquid—Liquid Extraction—Apparatus. For example, the choice may be for single- or multi-staged, preferably single-staged, extractions, and also for those in cocurrent or countercurrent mode and preferably in countercurrent mode.

Preference is given to using sieve tray columns, arrangedly or randomly packed columns, stirred vessels or mixer-settler apparatuses and also pulsed columns or columns having rotating internals.

The prewash e) is preferably used whenever metal salts and preferably copper or copper salts are (concomitantly) used as inhibitors.

An afterwash g) may be preferable to remove traces of base or salt traces from the reaction mixture neutralized in f).

By way of neutralization f), the reaction mixture which may optionally have been prewashed and which may still comprise small amounts of catalyst and the main amount of excess (meth)acrylic acid can be neutralized with a 5-25%, preferably 5-20% and more preferably 5-15% by weight aqueous solution of a base, for example alkali metal or alkaline earth metal oxides, hydroxides, carbonates or bicarbonates, preferably aqueous sodium hydroxide solution, aqueous potassium hydroxide solution, sodium bicarbonate, sodium carbonate, potassium bicarbonate, calcium hydroxide, milk of lime, ammonia gas, ammonia water or potassium carbonate, to which solution 5-15% by weight of sodium chloride, potassium chloride, ammonium chloride or ammonium sulfate may optionally have been added, more preferably with aqueous sodium hydroxide solution or aqueous sodium hydroxide-sodium chloride solution. The degree of neutralization is preferably in the range from 5 to 60 mol %, preferably in the range from 10 to 40 mol %, more preferably in the range from 20 to 30 mol %, based on the acid-functional monomers. This neutralization can take place before and/or during the polymerization, preferably before the polymerization. Another preferred degree of neutralization is in the range from 50 to 100 mol %, more preferably 55-80 mol %, especially from 60 to 75 mol %. Preferably, the crosslinker solution in pure acrylic acid is mixed together with the acrylate solution just before the polymerization to set the degree of neutralization.

The base is added in such a way that the temperature in the apparatus does not rise above 60° C. and is preferably in the range from 20 to 35° C., and the pH is 4-13, preferably 4.5-10. The heat of neutralization is preferably removed by cooling the vessel with the aid of internal cooling coils or via jacketed cooling.

The ratio of reaction mixture to neutralizing liquor is generally in the range from 1:0.1 to 1:1, preferably in the range from 1:0.2 to 1:0.8 and more preferably in the range from 1:0.3 to 1:0.7.

With regard to the apparatus, the above statements apply.

h) When the reaction mixture comprises a solvent, it may be substantially removed by distillation. Preferably, any solvent present is removed from the reaction mixture after washing and/or neutralization, but if desired this may also be done prior to the wash or neutralization.

For this, the reaction mixture is admixed with an amount of storage stabilizer, preferably hydroquinone monomethyl ether, such that, after removal of the solvent, 100-500, preferably 200-500 and more preferably 200-400 ppm thereof are comprised in the target ester (residue). But instead of hydroquinone monomethyl ether it is also possible to use the above-described sterically hindered phenols preferably alone or mixed with other stabilizers.

The distillative removal of the main amount of solvent is effected for example in a stirred tank with jacketed heating and/or internal heating coils under reduced pressure, for example at 20-700 mbar, preferably 30-500 mbar and more preferably 50-150 mbar and 40-80° C.

It will be appreciated that the distillation can also be accomplished in a falling-film or thin-film evaporator. For this, the reaction mixture is recirculated, preferably two or more times, through the apparatus under reduced pressure, for example at 20-700 mbar, preferably 30-500 mbar and more preferably 50-150 mbar and 40-80° C.

An inert gas, preferably an oxygen-containing gas, more preferably air or a mixture of air and nitrogen (lean air) may preferably be introduced into the distillation apparatus, for example 0.1-1, preferably 0.2-0.8 and more preferably 0.3-0.7 m³/m³h, based on the volume of the reaction mixture.

The residual solvent content of the residue is generally below 5% by weight, preferably 0.5-5% and more preferably 1-3% by weight after the distillation.

The removed solvent is condensed and preferably reused.

If necessary, a solvent stripping operation i) can be carried out in addition to or in lieu of the distillation.

For this, the target ester, which still comprises small amounts of solvent, is heated to 50-90° C. and preferably 80-90° C. and the remaining amounts of solvent are removed with a suitable gas in a suitable apparatus. There are circumstances where a vacuum can be applied in support, if desired.

Examples of useful apparatuses include columns of conventional design which contain conventional internals, for example trays, dumped packing or structured packing, preferably dumped packing. Useful column internals include in principle all common internals, for example trays, arranged packing and/or random packing. Preferred trays include bubble trays, sieve trays, valve trays, Thormann trays and/or dual-flow trays, while preferred dumped packings are those of rings, coils, saddles, Raschig, Intos or Pall rings, barrel or Intalox saddles, Top-Pak, etc. or braids.

Another possibility here is a falling-film, thin-film or wipe-film evaporator, for example a Luwa, Rotafilm or Sambay evaporator, which may be splash-guarded with a demister for example.

Useful gases include gases which are inert under the stripping conditions, preferably oxygen-containing gases, more preferably air or mixtures of air and nitrogen (lean air) or water vapor, especially such gases which have been preheated to 50-100° C.

The stripping gas rate is for example in the range from 5 to 20, more preferably in the range from 10 to 20 and most preferably in the range from 10 to 15 m³/m3h, based on the volume of the reaction mixture.

If necessary, the ester can be subjected to a filtration j) at any stage of the workup process, preferably after washing/neutralization and any effected solvent removal, in order that precipitated traces of salts and any decolorizing agent present may be removed.

In a conceivable embodiment, the esterification a) of alkoxylated glycol with (meth)acrylic acid in the presence of at least one esterification catalyst C and of at least one polymerization inhibitor D is carried out in a molar excess of at least 10:1, as indicated above, without a solvent capable of forming an azeotrope with water.

In a preferred embodiment the excess (meth)acrylic acid is preferably substantially not removed, ie only that fraction of (meth)acrylic acid is removed from the reaction mixture that is determined by the volatility at the employed temperature, and beyond that no measures are carried out to remove the carboxylic acid, for example no distillative, rectificative, extractive (washing for example), absorptive (for example passing through activated carbon or through ion exchangers) and/or chemical steps such as scavenging of the carboxylic acid with epoxides are carried out.

The extent to which the (meth)acrylic acid in the reaction mixture is removed from it is preferably not more than 75% by weight, more preferably not more than 50% by weight, even more preferably not more than 25% by weight, especially not more than 10% by weight and most preferably not more than 5% by weight, based on the (meth)acrylic acid in the reaction mixture after the reaction has ended. In a particularly preferred embodiment, stage b) can be omitted, so that only the fraction of water of reaction and (meth)acrylic acid is removed from the reaction mixture that is determined by the volatility at the employed temperature. This can preferably be prevented by substantially complete condensation.

Furthermore, the esterification catalyst C used is likewise substantially left in the reaction mixture.

The DIN EN 3682 acid number of the reaction mixture thus obtainable is preferably at least 25 mg of KOH/g of reaction mixture, more preferably at least 35 mg of KOH/g, even more preferably at least 45 mg of KOH/g. The acid number is more preferably in the range from 25 to 80 and most preferably in the range from 35 to 50 mg of KOH/g.

Any pre- or afterwash e) or g) is preferably omitted; merely a filtration step j) can be sensible.

The reaction mixture can subsequently be diluted in step c), in which case it is preferably converted within 6 hours and more preferably within 3 hours to the hydrogel. It may preferably be neutralized in a step f).

The order of the steps c), j) and f) is arbitrary.

The present invention further provides a composition of matter comprising

-   -   at least one ester F obtainable by one of the esterification         processes described above,     -   (meth)acrylic acid and     -   diluent G.

The composition of matter of the present invention may further comprise

-   -   esterification catalyst C in protonated or unprotonated form,     -   polymerization inhibitor D and also     -   optionally solvent E if used in the esterification.

The composition of matter may optionally have been neutralized and have a pH as cited above under f).

When the composition of matter has been neutralized, at least a portion of the (meth)acrylic acid has been converted into their water-soluble alkali metal, alkaline earth metal or ammonium salts.

A preferred composition of matter comprises

-   -   ester F in a fraction from 0.1% to 40% by weight, more         preferably from 0.5% to 20%, even more preferably from 1% to         10%, especially from 2% to 5% and specifically from 2% to 4% by         weight,     -   monomer M at 0.5-99.9% by weight, more preferably 0.5-50% by         weight, even more preferably 1-25%, especially 2-15% and         specifically from 3% to 8% or 4-6% by weight,     -   esterification catalyst C at 0-10% by weight, more preferably         0.02-5%, even more preferably 0.05-2.5% by weight and especially         0.1-1% by weight,     -   polymerization inhibitor D at 0-5% by weight, more preferably         0.01-1.0%, even more preferably 0.02-0.75%, especially 0.05-0.5%         and specifically 0.075-0.25% by weight,     -   solvent E at 0-10% by weight, more preferably 0-5% by weight,         even more preferably 0.05-1.5% by weight and especially 0.1-0.5%         by weight, with the proviso that the sum total is always 100% by         weight, and also     -   optionally diluent G ad 100% by weight.

The reaction mixtures obtainable by the above process and compositions of matter according to the present invention can find use

-   -   as a free-radical crosslinker of water-absorbing hydrogels,     -   as a starting material for producing polymer dispersions,     -   as a starting material for producing polyacrylates (except         hydrogels),     -   as a paint raw material or     -   as a cement additive.

Compositions of matter according to the present invention which are particularly useful as radical crosslinkers of water-absorbing hydrogels have a solubility in distilled water at 25° C. of not less than 0.5% by weight, preferably not less than 1% by weight, more preferably not less than 2% by weight, even more preferably not less than 5% by weight, particularly preferably not less than 10% by weight, even more preferably not less than 20% by weight and especially not less than 30% by weight.

k) The reaction mixture from the esterification, including workup steps thereof, where practiced, for example the reaction mixture from f) or, when f) is omitted, from b) or, when b) is omitted, the reaction mixture from a), can be admixed optionally with additional monoethylenically unsaturated compounds N which bear no acid groups but are copolymerizable with the hydrophilic monomers M and can then be polymerized in the presence of at least one radical initiator K and optionally at least one grafting base L to prepare water-absorbing hydrogels.

It may be preferable

l) to postcrosslink the reaction mixture of k).

Useful hydrophilic monomers M for preparing k) these highly swellable hydrophilic hydrogels include for example acids capable of addition polymerization, such as acrylic acid, methacrylic acid, ethacrylic acid, α-chloroacrylic acid, crotonic acid, maleic acid, maleic anhydride, vinylsulfonic acid, vinylphosphonic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, allylsulfonic acid, sulfoethyl acrylate, sulfomethacrylate, sulfopropyl acrylate, sulfopropyl methacrylate, 2-hydroxy-3-acryloyloxypropylsulfonic acid, 2-hydroxy-3-methacryloyloxypropylsulfonic acid, allylphosphonic acid, styrenesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-acrylamido-2-methylpropane-phosphonic acid and also their amides, hydroxyalkyl esters and amino- or ammonio-containing esters and amides. These monomers can be used alone or mixed with each other. Furthermore water-soluble N-vinylamides and also diallyldimethylammonium chloride. Preferred hydrophilic monomers are compounds of the formula V

where

-   -   R³ is hydrogen, methyl or ethyl,     -   R⁴ is —COOR⁶, a sulfonyl group, a phosphonyl group, a         (C₁-C₄)-alkanol-esterified phosphonyl group or a group of the         formula VI     -   R⁵ is hydrogen, methyl, ethyl or a carboxyl group,     -   R⁶ is hydrogen, amino or hydroxy-(C₁-C₄)-alkyl and     -   R⁷ is a sulfonyl group, a phosphonyl group or a carboxyl group.

Examples of (C₁-C₄)-alkanols are methanol, ethanol, n-propanol and n-butanol.

Particularly preferred hydrophilic monomers are acrylic acid and methacrylic acid, especially acrylic acid.

To optimize properties, it can be sensible to use additional monoethylenically unsaturated compounds N which do not bear an acid group but are copolymerizable with the monomers bearing acid groups. Such compounds include for example the amides and nitriles of monoethylenically unsaturated carboxylic acids, for example acrylamide, methacrylamide and N-vinylformamide, N-vinylacetamide, N-methylvinyl-acetamide, acrylonitrile and methacrylonitrile. Examples of further suitable compounds are vinyl esters of saturated C₁- to C₄-carboxylic acids such as vinyl formate, vinyl acetate or vinyl propionate, alkyl vinyl ethers having at least 2 carbon atoms in the alkyl group, for example ethyl vinyl ether or butyl vinyl ether, esters of monoethylenically unsaturated C₃- to C₆-carboxylic acids, for example esters of monohydric C₁- to C₁₈-alcohols and acrylic acid, methacrylic acid or maleic acid, monoesters of maleic acid, for example methyl hydrogen maleate, N-vinyllactams such as N-vinylpyrrolidone or N-vinylcaprolactam, acrylic and methacrylic esters of alkoxylated monohydric saturated alcohols, for example of alcohols having from 10 to 25 carbon atoms which have been reacted with from 2 to 200 mol of ethylene oxide and/or propylene oxide per mole of alcohol, and also monoacrylic esters and monomethacrylic esters of polyethylene glycol or polypropylene glycol, the molar masses (M_(n)) of the polyalkylene glycols being up to 2000, for example. Further suitable monomers are styrene and alkyl-substituted styrenes such as ethylstyrene or tert-butylstyrene.

These monomers without acid groups may also be used in mixture with other monomers, for example mixtures of vinyl acetate and 2-hydroxyethyl acrylate in any proportion. These monomers without acid groups are added to the reaction mixture in amounts within the range from 0 to 50% by weight, preferably less than 20% by weight.

The crosslinked (co)polymers preferably consist of acid-functional monoethylenically unsaturated monomers which have optionally been converted into their alkali metal or ammonium salts before or after polymerization and of 0-40% by weight based on their total weight of monoethylenically unsaturated monomers which do not bear add groups.

The production, testing and use of (meth)acrylic acid (co)polymers, polyacrylic acids and superabsorbents has been extensively described before and therefore is well known, see for example “Modern Superabsorbent Polymer Technology”, F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998 or Markus Frank “Superabsorbents” in Ullmann's Handbuch der technischen Chemie, Volume 35, 2003.

Preference is given to such hydrogels which are obtained by crosslinking addition polymerization or copolymerization of acid-functional monoethylenically unsaturated monomers M or salts thereof.

The polymers obtainable are notable for an improved saponification index (VSI).

In the postcrosslinking process, the starting polymer is treated with a postcrosslinker and preferably during or after the treatment postcrosslinked and dried by raising the temperature, the crosslinker preferably being comprised in an inert solvent. Inert solvents are solvents which substantially do not react either with the starting polymer or with the postcrosslinker. Preference is given to such solvents which do not react chemically with the starting polymer or with the postcrosslinker to an extent of more than 90%, preferably more than 95%, more preferably more than 99% and especially more than 99.5%.

Postcrosslinking l) and drying m) is preferably carried out at from 30 to 250° C., especially 50-200° C. and most preferably at from 100 to 180° C. The surface postcrosslinking solution is preferably applied by spraying the polymer in suitable spray mixers. After spraying, the polymer powder is thermally dried, and the crosslinking reaction can take place not only before but also during the drying operation. Preference is given to spraying a solution of the crosslinker in reaction mixers or mixing and drying ranges such as for example Lödige mixers, BEPEX mixers, NAUTA mixers, SHUGGI mixers or PROCESSALL. It is moreover also possible to use fluidized bed dryers.

The drying operation can take place in the mixer itself, by heating of the shell or by blowing in hot air. Also suitable is a downstream dryer such as for example a shelf dryer, a rotary tube oven or a heatable screw. But it is also possible to utilize an azeotropic distillation as drying technique, for example. The preferred residence time at this temperature in the reaction mixer or dryer is below 60 min and more preferably below 30 min.

Preference is given to the above processes wherein the starting polymer is a polymeric acrylic acid or a polyacrylate, especially a polymeric acrylic acid or a polyacrylate obtained by free-radical polymerization using a polyfunctional ethylenically unsaturated radical crosslinker.

Preference is given to such processes wherein the composition of matter comprising radical crosslinkers, ie the ester F, and diluents G in a ratio of 0.1-20% by weight and especially 0.5-10% by weight based on the mass of the starting polymer is used.

Preference is given to such processes wherein the radical crosslinker is used in a dose of 0.01-5.0% by weight, preferably 0.02-3.0% by weight, more preferably 0.03-2.5% by weight, especially 0.05-1.0% and specifically from 0.1% to 0.75% by weight based on the starting polymer.

The present invention also provides polymers prepared by one of the processes mentioned above and for their use in hygiene artides, packaging materials and nonwovens and also for the use of an abovementioned composition of matter for producing crosslinked or thermally crosslinkable polymers, especially in paints and varnishes.

The highly swellable hydrophilic hydrogels to be used (starting polymers) are in particular polymers of (co)polymerized hydrophilic monomers M, graft (co)polymers of one or more hydrophilic monomers M on a suitable grafting base L, crosslinked cellulose or starch ethers or natural products capable of swelling in aqueous fluids, for example guar derivatives. These hydrogels are known to one skilled in the art and are described for example in U.S. Pat. No. 4,286,082, DE-C-27 06 135, U.S. Pat. No. 4,340,706, DE-C-37 13 601, DE-C-28 40 010, DE-A-43 44 548, DE-A-40 20 780, DE-A-40 15 085, DE-A-39 17 846, DE-A-38 07 289, DE-A-35 33 337, DE-A-35 03 458, DE-A-42 44 548, DE-A-42 19 607, DE-A-40 21 847, DE-A-38 31 261, DE-A-35 11 086, DE-A-31 18 172, DE-A-30 28 043, DE-A-44 18 881, EP-A-0 801 483, EP-A-0 455 985, EP-A-0 467 073, EP-A-0 312 952, EP-A-0 205 874, EP-A-0 499 774, DE-A-26 12 846, DE-A-40 20 780, EP-A-0 205 674, U.S. Pat. No. 5,145,906, EP-A-0 530 438, EP-A-0 670 073, U.S. Pat. No. 4,057,521, U.S. Pat. No. 4,062,817, U.S. Pat. No. 4,525,527, U.S. Pat. No. 4,295,987, U.S. Pat. No. 5,011,892, U.S. Pat. No. 4,076,663 or U.S. Pat. No. 4,931,497. Also of particular suitability are highly swellable hydrogels from a manufacturing operation as described in WO 01/38402 and also highly swellable inorganic/organic hybrid hydrogels as described in DE 198 54 575. The content of the aforementioned patent documents, especially the hydrogels obtained by the processes, is incorporated herein by reference.

Suitable grafting bases L for hydrophilic hydrogels obtainable by graft copolymerization of olefinically unsaturated acids can be of natural or synthetic origin. Examples are starch, cellulose, cellulose derivatives and also other polysaccharides and oligosaccharides, polyalkylene oxides, especially polyethylene oxides and polypropylene oxides, and also hydrophilic polyesters.

The water-absorbing polymer is obtainable by free-radical graft copolymerization of acrylic acid or acrylate onto a water-soluble polymer matrix. Nonlimiting examples of suitable water-soluble polymer matrices are alginates, polyvinyl alcohol and polysaccharides such as starch for example. A graft copolymerization for the purposes of the present invention utilizes a polyfunctional ethylenically unsaturated radical crosslinker.

The water-absorbing polymer can be an organic/inorganic hybrid polymer formed from a polymeric acrylic acid or polyacrylate on the one hand and a silicate, aluminate or aluminosilicate on the other. More particularly, the polymeric acrylic acid or polyacrylate used may be obtained by free-radical polymerization using a polyfunctional ethylenically unsaturated radical crosslinker and formed using a water-soluble silicate or soluble aluminate or mixture thereof.

Preferred hydrogels are in particular polyacrylates, polymethacrylates and also the U.S. Pat. No. 4,931,497, U.S. Pat. No. 5,011,892 and U.S. Pat. No. 5,041,496 graft polymers. Very particularly preferred hydrogels are the kneader polymers described in WO 01/38402 and the polyacrylate-based organic/inorganic hybrid hydrogels described in DE 198 545 75.

The substances prepared according to the present invention, which are useful as radical crosslinkers in hydrogels, can be used alone or in combination with other crosslinkers, for example internal or surface crosslinkers, for example the following:

Suitable further crosslinkers are in particular methylenebisacrylamide, methylene-bismethacrylamide, esters of unsaturated mono- or polycarboxylic acids with polyols, such as diacrylate, triacrylate or tetraacrylate, for example butanediol diacrylate, butanediol dimethacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, and also trimethylolpropane triacrylate or glycerol diacrylate and glycerol triacrylate or pentaerythritol tetraacrylate and allyl compounds such as allyl(meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allyl esters of phosphoric acid and also vinylphosphonic acid derivatives as described for example in EP-A-0 343 427. However, particular preference for use in the process of the present invention is given to hydrogels prepared using polyallyl ethers as further crosslinkers and by acidic homopolymerization of acrylic acid. Suitable crosslinkers are pentaerythritol triallyl ether, pentaerythritol tetraallyl ether, trimethylolpropane diallyl ether, polyethylene glycol diallyl ether, monoethylene glycol diallyl ether, glycerol diallyl ether, glycerol triallyl ether, polyallyl ethers based on sorbitol and also ethoxylated variants thereof. Particularly preferred crosslinkers further include polyethylene glycol diacrylates, ethoxylated derivatives of trimethylolpropane triacrylate, for example Sartomer SR 9035, and also ethoxylated derivatives of glycerol diacrylate and glycerol triacrylate. It is obviously also possible to use mixtures of the above crosslinkers.

Particular preference is given to combinations of crosslinkers wherein further crosslinkers can be dispersed in the inventive crosslinkers F. Examples of such crosslinker combinations are the inventive crosslinkers F together with di- or tripropylene glycol diacrylate and propoxylated glycerol triacrylates. Further examples of such crosslinker combinations are the inventive crosslinkers together with butanediol diacrylate or trimethylolpropane triacrylate or pentaerythritol triallyl ether.

Very particular preference is given to hydrogels prepared using an ester F prepared according to the present invention as a radical crosslinker.

The water-absorbing polymer is preferably a polymeric acrylic acid or a polyacrylate. This water-absorbing polymer can be prepared by a process known from the literature. Preference is given to polymers which comprise crosslinking comonomers (0.001-10 mol %), but very particular preference is given to polymers which were obtained by free-radical polymerization and where a polyfunctional ethylenically unsaturated radical crosslinker was used. The inventive ester mixtures preferably permit each ester component F_(i) to be used at less than 2% by weight and preferably 1% by weight based on the total amount of the monomers. It is particularly preferable for the sum total of all ester components to be below 2% by weight and preferably below 1% by weight.

The highly swellable hydrophilic hydrogels are preparable by addition polymerization processes known per se. Preference is given to the addition polymerization in aqueous solution conducted as a gel polymerization. It involves, as stated above, dilute, preferably aqueous and more preferably 15-50% by weight aqueous, solutions of one or more hydrophilic monomers and optionally of a suitable grafting base L being polymerized in the presence of a free-radical initiator by utilizing the Trommsdorff-Norrish effect (Makromol. Chem. 1, 169 (1947)) preferably without mechanical mixing. The polymerization reaction may be carried out at from 0° C. to 150° C., and preferably at from 10° C. to 100° C., not only at atmospheric pressure but also at superatmospheric or reduced pressure. Typically, the polymerization can also be carried out in a protective gas atmosphere, preferably under nitrogen. The addition polymerization may be induced using high-energy electromagnetic rays or the customary chemical polymerization initiators K, for example organic peroxides, such as benzoyl peroxide, tert-butyl hydroperoxide, methyl ethyl ketone peroxide, cumene hydroperoxide, azo compounds such as azobisisobutyronitrile and also inorganic peroxy compounds such as (NH₄)₂S₂O₈, K₂S₂O₈ or H₂O₂.

They can optionally be used in combination with reducing agents such as ascorbic acid, sodium hydrogensulfite and iron(II) sulfate or redox systems where the reducing component comprised is an aliphatic and aromatic sulfinic acid, such as benzenesulfinic acid and toluenesulfinic acid or derivatives thereof, for example Mannich adducts of sulfinic acids, aldehydes and amino compounds, as described in DE-C-1 301 566. The performance properties of the polymers can be further improved by postheating the polymer gels in the temperature range from 50° to 130° C. and preferably from 70° to 100° C. for several hours.

The gels obtained are neutralized to the extent of 0-100 mol %, preferably 25-100 mol % and more preferably 50-5 mol % based on monomer used, for which the customary neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides or the corresponding alkali metal carbonates, but more preferably sodium hydroxide, sodium carbonate and sodium bicarbonate. When sodium hydroxide is used, it is particularly preferable to use the grade obtainable by membrane electrolysis.

Neutralization is typically achieved by mixing the neutralizing agent as an aqueous solution or else preferably as a solid into the gel. For this, the gel is mechanically comminuted, for example by means of a meat grinder, and the neutralizing agent is sprayed on, scattered on or poured on and then carefully mixed in. The gel mass obtained can then be repeatedly passed through the meat grinder for homogenization. The neutralized gel mass is then dried with a belt or can dryer until the residual moisture content is preferably below 10% by weight and especially below 5% by weight.

The addition polymerization as such can also be carried out by any other process described in the literature. More particularly, the neutralization of the acrylic acid can also be carried out prior to the polymerization, as described above in step f).

Irrespectively of whether neutralization takes place before, during or after the polymerization, it is particularly preferable to use acrylic acid having a dimer content below 2000 ppm, more preferably below 1000 ppm and most preferably below 500 ppm. Very particular preference is given to using sodium hydroxide from a membrane electrolysis. This is notable for its high purity (for example low chloride content and absence of mercury traces) compared with other processes. It is of course also possible to use sodium hydroxide from the amalgam or diaphragm process. The polymerization can then be carried out in a conventional belt reactor or a kneading reactor continuously or else batchwise. When the polymerization is carried out in a belt reactor, initiation by electromagnetic radiation and preferably by UV radiation or alternatively initiation by means of a redox initiator system is particularly preferred. Very particular preference is also given to a combination of the two methods of initiation: electromagnetic radiation and chemical redox initiator system simultaneously.

n) The dried hydrogel can then be ground and sieved, in which case it is customary to use roll mills, pin mills or sieving mills for the grinding. The preferred particle size of the sieved hydrogel is preferably in the range 45-1000 μm, more preferably at 45-850 μm, even more preferably at 200-850 μm, and most preferably at 300-850 μm. A further particularly preferred range is 150-850 μm, especially 150-700 μm and more preferably 200-600 μm and most preferably 150-550 μm. A further particular range is 200-800 μm and a particularly preferred range is 250-650 μm, a very particularly preferred range is 300-600 μm. Particularly preferred ranges are 200-500 μm, 100-450 μm and also 150-400 μm. These ranges preferably cover 80% by weight of the particles and especially 90% by weight of the particles. The size distribution can be determined using established sieving methods or else preferably using optical methods (photographs).

The present invention further provides crosslinked hydrogels which comprise at least one hydrophilic monomer M in copolymerized form and have been crosslinked using an ester F of polyol with (meth)acrylic add. The ester can be prepared in a manner according to the present invention or in a prior art manner and is preferably prepared in a manner according to the present invention.

Useful esters F include compounds as described above.

The CRC value [g/g] of the hydrogel-forming polymers according to the present invention can be measured by the methods indicated in the description and is preferably above 10, especially 11, 12, 13, 14, 15, 16, 18, 20, 22, 24, or higher, more preferably 25, especially 26, 27, 28, 29, even more preferably 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45 or higher.

The AUL 0.7 psi value [g/g] of the hydrogel-forming polymers according to the present invention can be measured by the methods indicated in the description part and is preferably above 8, especially 9, 10, 11, 12, 13,14 or higher, more preferably 15, especially 16, 17, 18, 19, or higher, even more preferably above 20, especially 21, 22, 23, 24, 25, 26, 27, 28, or higher.

The AUL 0.5 psi value [g/g] of the hydrogel-forming polymers according to the present invention can be measured by the methods indicated in the description part and is preferably above 8, especially 9, 10, 11, 12, 13, 14 or higher, more preferably 15, especially 16, 17, 18, 19, or higher, even more preferably above 20, especially 21, 22, 23, 24, 25, 26, 27, 28, or higher.

The saponification index VSI of the hydrogel-forming polymers according to the present invention can be measured by the methods indicated in the description part and is preferably less than 10, especially 9.5, 9 or 8.5 or lower, more preferably less than 8, especially 7.5, 7, 6.5, 6, 5.5 or lower, even more preferably less than 5, especially 4.5, 4, 3.5 or lower.

The residual crosslinker content of the inventive hydrogel-forming polymers can be measured by the methods indicated in the description part and is preferably less than 30 ppm, more preferably less than 20 ppm, most preferably less than 10 ppm, especially 9.5 ppm, 9 ppm or 8.5 ppm or less, more preferably less than 8 ppm, especially 7.5 ppm, 7 ppm, 6.5 ppm, 6 ppm, 5.5 ppm or less, even more preferably less than 5 ppm, especially 4.5 ppm, 4 ppm, 3.5 ppm or less. When a plurality of crosslinkers are used in the mixture, these maximum values relate to each individual crosslinker in the mixture.

Deployment and Use of the Hydrogel-Forming Polymers According to the Present Invention

The present invention further relates to the use of the abovementioned hydrogel-forming polymers in hygiene articles comprising

(P) a liquid-pervious topsheet

(Q) a substantially liquid-impervious backsheet

(R) a core positioned between (P) and (Q) and comprising

10-100% by weight of the hydrogel-forming polymer according to the present invention

0-90% by weight of hydrophilic fiber material

preferably 20-100% by weight of the hydrogel-forming polymer according to the present invention, 0-80% by weight of hydrophilic fiber material

more preferably 30-100% by weight of the hydrogel-forming polymer according to the present invention, 0-70% by weight of hydrophilic fiber material

even more preferably 40-100% by weight of the hydrogel-forming polymer according to the present invention, 0-60% by weight of hydrophilic fiber material

yet even more preferably 50-100% by weight of the hydrogel-forming polymer according to the present invention, 0-50% by weight of hydrophilic fiber material

particularly preferably 60-100% by weight of the hydrogel-forming polymer according to the present invention, 0-40% by weight of hydrophilic fiber material

especially preferably 70-100% by weight of the hydrogel-forming polymer according to the present invention, 0-30% by weight of hydrophilic fiber material

extremely preferably 80-100% by weight of the hydrogel-forming polymer according to the present invention, 0-20% by weight of hydrophilic fiber material

most preferably 90-100% by weight of the hydrogel-forming polymer according to the present invention, 0-10% by weight of hydrophilic fiber material

(S) optionally a tissue layer positioned directly above and below said core (R), and

(T) optionally an acquisition layer positioned between (P) and (R).

The percentages are to be understood so that in the case of 10-100% by weight, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% up to in each case 100% by weight of hydrogel-forming polymer according to the present invention and all intermediate % (for example 12.2%) are possible and correspondingly hydrophilic fiber material from 0% to in each case 89%, 88%, 87%, 86%, 85%, 83%, 82%, 81 % by weight and intermediate percentages (for example 87.8%) are possible. When further materials are present in the core, the percentages of polymer and fiber decrease accordingly. The same applies to the preferred ranges, for example in the case of extremely preferable 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89% by weight can be present for the hydrogel-forming polymer according to the present invention and correspondingly 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11% by weight for the fiber material. Thus, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29% to 100% by weight of the hydrogel-forming polymer according to the present invention can be present in the preferred range, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39% to 100% by weight can be present for the hydrogel-forming polymer according to the present invention, in the more preferred range, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49% to 100% by weight can be present for the hydrogel-forming polymer according to the present invention, in the even more preferred range, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59% to 100% by weight can be present for the hydrogel-forming polymer according to the present invention, in the yet even more preferred range, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69% to 100% by weight can be present for the hydrogel-forming polymer according to the present invention, in the particularly preferred range, 70%, 71%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79% to 100% by weight can be present for the hydrogel-forming polymer according to the present invention in the especially preferred range, and 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% by weight can be present for the hydrogel-forming polymer according to the present invention in the most preferred range.

Hygiene articles for the purposes of the present invention include not only incontinence pads and incontinence briefs for adults but also diapers for babies.

The liquid-pervious topsheet (P) is the layer which is in direct contact with the skin of the wearer. Its material comprises customary synthetic or manufactured fibers or films of polyesters, polyolefins, rayon or natural fibers such as cotton. In the case of non-woven materials the fibers are generally joined together by binders such as polyacrylates. Preferred materials are polyesters, rayon and blends thereof, polyethylene and polypropylene. Examples of liquid-pervious layers are described in WO 99/57355 A1, EP 102 388 3 A2.

The liquid-impervious layer (Q) is generally a sheet of polyethylene or polypropylene. However, it is possible to use any other materials which can be processed into liquid-impervious sheets. Liquid impervious in this connection means an imperviousness to condensed liquids. At the same time, however, the sheet can exhibit perviousness to the vapor of the liquid, and modern diaper constructions often combine a high vapor perviousness with maximum imperviousness to the underlying condensed liquid, which is generally water or urine.

The core (R) comprises not only the hydrogel-forming polymer according to the present invention but also hydrophilic fiber material. By hydrophilic is meant that aqueous fluids spread quickly over the fiber. The fiber material is usually cellulose, modified cellulose, rayon, polyester such as polyethylene terephthalate. Particular preference is given to cellulose fibers such as pulp. The fibers generally have a diameter of 1-200 μm and preferably 10-100 μm, and also have a minimum length of 1 mm.

Diaper construction and shape is common knowledge and described for example in WO 95/26 209 page 66 line 34 to page 69 line 11, DE 196 04 601 A1, EP-A-0 316 518 and EP-A-0 202 127. Diapers and other hygiene articles are generally also described in WO 00/65084, especially at pages 6-15, WO 00/65348, especially at pages 4-17, WO 00/35502, especially pages 3-9, DE 19737434, WO 98/8439. Hygiene articles for feminine care are described in the following references. The subject hydrogel-forming polymers capable of absorbing aqueous fluids can be used there. Feminine care references: WO 95/24173: Absorption Article for Controlling Odour, WO 91/11977: Body Fluid Odour Control, EP 389023: Absorbent Sanitary Articles, WO 94/25077: Odour Control Material, WO 97/01317: Absorbent Hygienic Article, WO 99/18905, EP 834297, U.S. Pat. No. 5,762,644, U.S. Pat. No. 5,895,381, WO 98/57609, WO 2000/065083, WO 2000/069485, WO 2000/069484, WO 2000/069481, U.S. Pat. No. 6,123,693, EP 1104666, WO 2001/024755, WO 2001/000115, EP 105373, WO 2001/041692, EP 1074233. Tampons are described in the following references: WO 98/48753, WO 98/41179, WO 97/09022, WO 98/46182, WO 98/46181, WO 2001/043679, WO 2001/043680, WO 2000/061052, EP 1108408, WO 2001/033962, DE 200020662, WO 2001/001910, WO 2001/001908, WO 2001/001909, WO 2001/001906, WO 2001/001905, WO 2001/24729. Incontinence articles are described in the following references: Disposable Absorbent Article for Incontinent Individuals: EP 311344 description pages 3-9; Disposable Absorbent Article: EP 850623; Absorbent Article: WO 95/26207; Absorbent Article: EP 894502; Dry Laid Fibrous Structure: EP 850 616; WO 98/22063; WO 97/49365; EP 903134; EP 887060; EP 887059; EP 887058; EP 887057; EP 887056; EP 931530; WO 99/25284; WO 98/48753. Feminine care and incontinence articles are described in the following references: Catamenial Device: WO 93/22998 description pages 26-33; Absorbent Members for Body Fluids: WO 95/26209 description pages 36-69; Disposable Absorbent Article: WO 98/20916 description pages 13-24; Improved Composite Absorbent Structures: EP 306262 description pages 3-14; Body Waste Absorbent Article: WO 99/45973. These references and the references therein are hereby expressly incorporated herein.

The hydrogel-forming polymers according to the present invention are very useful as absorbents for water and aqueous fluids, so that they may be used with advantage as a water retainer in market gardening, as a filter aid and particularly as an absorbent component in hygiene articles such as diapers, tampons or sanitary napkins.

Incorporation and Fixation of the Highly Swellable Hydrogels According to the Present Invention

In addition to the above-described highly swellable hydrogels, the absorbent composition of the present invention includes constructions which comprise highly swellable hydrogels or to which they are fixed. Any construction is suitable that is capable of accommodating highly swellable hydrogels and of being integrated into the absorption layer. A multiplicity of such compositions is already known and described in detail in the literature. A construction for installing the highly swellable hydrogels can be for example a fiber matrix consisting of a cellulose fiber mixture (air-laid web, wet laid web) or synthetic polymer fibers (meltblown web, spunbonded web) or else of a fiber blend of cellulose fibers and synthetic fibers. Possible fiber materials are detailed in the chapter which follows. The air-laid web process is described for example in WO 98/28 478. Furthermore, open-celled foams or the like may be used to install highly swellable hydrogels.

Alternatively, such a construction can be the result of fusing two individual layers to form one or better a multiplicity of chambers which comprise the highly swellable hydrogels. Such a chamber system is described in detail in EP 0 615 736 A1 page 7 lines 26 et seq.

In this case, at least one of the two layers should be water pervious. The second layer may either be water pervious or water impervious. The layer material used may be tissues or other fabric, closed or open-celled foams, perforated films, elastomers or fabrics composed of fiber material. When the absorbent composition consists of a construction of layers, the layer material should have a pore structure whose pore dimensions are small enough to retain the highly swellable hydrogel particles. The above examples of the construction of the absorbent composition also include laminates composed of at least two layers between which the highly swellable hydrogels are installed and fixed.

Generally it is possible to fix hydrogel particles within the absorbent core to improve dry and wet integrity. Dry and wet integrity describes the ability to install highly swellable hydrogels into the absorbent composition in such a way that they withstand external forces not only in the wet but also in the dry state and highly swellable polymer does not dislocate or spill out. The forces referred to are especially mechanical stresses as occur in the course of moving about while wearing the hygiene article or else the weight pressure on the hygiene article in the case of incontinence especially. As to fixation, one skilled in the art knows a multiplicity of possibilities. Examples such as fixation by heat treatment, addition of adhesives, thermoplastics, binder materials are noted in WO 95/26 209 page 37 line 36 to page 41 line 14. The cited passage is thus part of this invention. Methods for enhancing wet strength are also to be found in WO 2000/36216 A1.

Furthermore, the absorbent composition may comprise a base material, for example a polymer film on which the highly swellable hydrogel particles are fixed. The fixing may be effected not only on one side but also on both sides. The base material can be water pervious or water impervious.

The above constructions of the absorbent composition incorporate the highly swellable hydrogels at a weight fraction of from 10-100% by weight, preferably 20-100% by weight, more preferably 30-100% by weight, even more preferably 40-100% by weight, much more preferably 50-100% by weight, particularly preferably 60-100% by weight, especially preferably 70-100% by weight, extremely preferably 80-100% by weight and most preferably 90-100% by weight, based on the total weight of the construction and of the highly swellable hydrogels.

Fiber Materials of the Absorbent Composition

The structure of the present absorbent composition according to the invention may be based on various fiber materials, which are used as a fiber network or matrices. The present invention includes not only fibers of natural origin (modified or unmodified) but also synthetic fibers.

A detailed overview of examples of fibers which can be used in the present invention is given in WO 95/26 209 page 28 line 9 to page 36 line 8. The cited passage is thus part of this invention.

Examples of cellulose fibers include cellulose fibers which are customarily used in absorption products, such as fluff pulp and cellulose of the cotton type. The materials (soft- or hardwoods), production processes such as chemical pulp, semichemical pulp, chemothermomechanical pulp (CTMP) and bleaching processes are not particularly restricted. For instance, natural cellulose fibers such as cotton, flax, silk, wool, jute, ethylcellulose and cellulose acetate are used.

Suitable synthetic fibers are produced from polyvinyl chloride, polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene chloride, polyacrylic compounds such as ORLON®, polyvinyl acetate, polyethyl vinyl acetate, soluble or insoluble polyvinyl alcohol. Examples of synthetic fibers include thermoplastic polyolefin fibers, such as polyethylene fibers (PULPEX®), polypropylene fibers and polyethylene-polypropylene bicomponent fibers, polyester fibers, such as polyethylene terephthalate fibers (DACRON® or KODEL®), copolyesters, polyvinyl acetate, polyethyl vinyl acetate, polyvinyl chloride, polyvinylidene chloride, polyacrylics, polyamides, copolyamides, polystyrene and copolymers of the aforementioned polymers and also bicomponent fibers composed of polyethylene terephthalate-polyethylene-isophthalate copolymer, polyethyl vinyl acetate/polypropylene, polyethylene/polyester, polypropylene/polyester, copolyester/polyester, polyamide fibers (nylon), polyurethane fibers, polystyrene fibers and polyacrylonitrile fibers. Preference is given to polyolefin fibers, polyester fibers and their bicomponent fibers. Preference is further given to thermally adhesive bicomponent fibers composed of polyolefin of the core-sheath type and side-by-side type on account of their excellent dimensional stability following fluid absorption.

The synthetic fibers mentioned are preferably used in combination with thermoplastic fibers. In the course of the heat treatment, the latter migrate to some extent into the matrix of the fiber material present and so constitute bond sites and renewed stiffening elements on cooling. Additionally the addition of thermoplastic fibers means that there is an increase in the present pore dimensions after the heat treatment has taken place. This makes it possible, by continuous addition of thermoplastic fibers during the formation of the absorbent layer, to continuously increase the fraction of thermoplastic fibers in the direction of the topsheet, which results in a similarly continuous increase in the pore sizes. Thermoplastic fibers can be formed from a multiplicity of thermoplastic polymers which have a melting point of less than 190° C., preferably in the range from 75° C. to 175° C. These temperatures are too low for damage to the cellulose fibers to be likely.

Lengths and diameters of the above-described synthetic fibers are not particularly restricted, and generally any fiber from 1 to 200 mm in length and from 0.1 to 100 denier (gram per 9000 meters) in diameter may preferably be used. Preferred thermoplastic fibers are from 3 to 50 mm in length, particularly preferred thermoplastic fibers are from 6 to 12 mm in length. The preferred diameter for the thermoplastic fiber is in the range from 1.4 to 10 decitex, and the range from 1.7 to 3.3 decitex (gram per 10 000 meters) is particularly preferred. The form of the fiber may vary; examples include woven types, narrow cylindrical types, cut/split yarn types, staple fiber types and continuous filament fiber types.

The fibers in the absorbent composition of the present invention can be hydrophilic and/or hydrophobic. According to the definition of Robert F. Gould in the 1964 American Chemical Society publication “Contact angle, wettability and adhesion”, a fiber is referred to as hydrophilic when the contact angle between the liquid and the fiber (or the fiber surface) is less than 90° or when the liquid tends to spread spontaneously on the same surface. The two processes are generally coexistent. Conversely, a fiber is termed hydrophobic when a contact angle of greater than 90° is formed and no spreading is observed.

Preference is given to using hydrophilic fiber material. Particular preference is given to using fiber material which is weakly hydrophilic on the body side and most hydrophilic in the region surrounding the highly swellable hydrogels. In the manufacturing process, layers having different hydrophilicities are used to create a gradient which channels impinging fluid to the hydrogel, where it is ultimately absorbed.

Suitable hydrophilic fibers for use in the absorbent composition of the present invention include for example cellulose fibers, modified cellulose fibers, rayon, polyester fibers, for example polyethylene terephthalate (DACRON®), and hydrophilic nylon (HYDROFIL®). Suitable hydrophilic fibers may also be obtained by hydrophilicizing hydrophobic fibers, for example the treatment of thermoplastic fibers obtained from polyolefins (e.g. polyethylene or polypropylene, polyamides, polystyrenes, polyurethanes, etc.) with surfactants or silica. However, for cost reasons and ease of availability, cellulosic fibers are preferred.

The highly swellable hydrogel particles are embedded into the fiber material described. This can be done in various ways, for example by using the hydrogel material and the fibers together to create an absorbent layer in the form of a matrix, or by incorporating highly swellable hydrogels into fiber mixture layers, where they are ultimately fixed, whether by means of adhesive or lamination of the layers.

The fluid-acquiring and -distributing fiber matrix may comprise synthetic fiber or cellulosic fiber or a mixture of synthetic fiber and cellulosic fiber, in which case the mixing ratio may vary from (100 to 0) synthetic fiber: (0 to 100) cellulosic fiber. The cellulosic fibers used may additionally have been chemically stiffened to increase the dimensional stability of the hygiene article.

The chemical stiffening of cellulosic fibers may be provided in different ways. A first way of providing fiber stiffening is by adding suitable coatings to the fiber material. Such additives include for example polyamide-epichlorohydrin coatings (Kymene® 557 H, Hercules, Inc. Wilmington, Del., USA), polyacrylamide coatings (described in U.S. Pat. No. 3,556,932 or as the Parez® 631 NC commercial product from American Cyanamid Co., Stamford, Conn., USA), melamine-formaldehyde coatings and polyethyleneimine coatings.

Cellulosic fibers may also be chemically stiffened by chemical reaction. For instance, suitable crosslinker substances may be added to effect crosslinking taking place within the fiber. Suitable crosslinker substances are typical substances used for crosslinking monomers including but not limited to C₂-C₈-dialdehydes, C₂-C₈-monoaldehydes having acid functionality and in particular C₂-C₉-polycarboxylic acids. Specific substances from this series are for example glutaraldehyde, glyoxal, glyoxylic acid, formaldehyde and citric acid. These substances react with at least 2 hydroxyl groups within any one cellulose chain or between two adjacent cellulose chains within any one cellulose fiber. The crosslinking causes a stiffening of the fibers, to which greater dimensional stability is imparted as a result of this treatment. In addition to their hydrophilic character, these fibers exhibit uniform combinations of stiffening and elasticity. This physical property makes it possible to retain the capillary structure even under simultaneous contact with fluid and compressive forces and to prevent premature collapse.

Chemically crosslinked cellulose fibers are known and described in WO 91/11162, U.S. Pat. No. 3,224,926, U.S. Pat. No. 3,440,135, U.S. Pat. No. 3,932,209, U.S. Pat. No. 4,035,147, U.S. Pat. No. 4,822,453, U.S. Pat. No. 4,888,093, U.S. Pat. No. 4,898,642 and U.S. Pat. No. 5,137,537. The chemical crosslinking imparts stiffening to the fiber material, which is ultimately reflected in improved dimensional stability for the hygiene article as a whole. The individual layers are joined together by methods known to one skilled in the art, for example intermelting by heat treatment, addition of hot-melt adhesives, latex binders, etc.

Methods of Making the Absorbent Composition

The absorbent composition is composed of constructions which comprise highly swellable hydrogels and the highly swellable hydrogels which are present in said constructions or fixed thereto.

Examples of processes to obtain an absorbent composition comprising for example a base material to which highly swellable hydrogels are fixed on one or both sides are known and included by the invention but not limited thereto.

Examples of processes to obtain an absorbent composition comprising for example a fiber material blend of synthetic fibers (a) and cellulosic fibers (b) embedded in highly swellable hydrogels (c), the blend ratio varying from (100 to 0) synthetic fiber: (0 to 100) cellulosic fiber, include (1) a process where (a), (b) and (c) are mixed together at one and the same time, (2) a process where a mixture of (a) and (b) is mixed into (c), (3) a process where a mixture of (b) and (c) is mixed with (a), (4) a process where a mixture of (a) and (c) is mixed into (b), (5) a process where (b) and (c) are mixed and (a) is continuously metered in, (6) a process where (a) and (c) are mixed and (b) is continuously metered in, and (7) a process where (b) and (c) are mixed separately into (a). Of these examples, processes (1) and (5) are preferred. The apparatus used in this process is not particularly restricted and any customary apparatus known to one skilled in the art can be used.

The absorbent composition obtained in this way can optionally be subjected to a heat treatment, so that an absorption layer having excellent dimensional stability in the moist state is obtained. The heat treatment process is not particularly restricted. Examples include heat treatment by feeding hot air or infrared irradiation. The temperature of the heat treatment is in the range from 60° C. to 230° C., preferably from 100° C. to 200° C., particularly preferably from 100° C. to 180° C.

The duration of the heat treatment depends on the type of synthetic fiber, its amount and the hygiene article production rate. Generally the duration of the heat treatment is in the range from 0.5 second to 3 minutes, preferably from 1 second to 1 minute.

The absorbent composition is generally provided for example with a liquid-pervious topsheet and a liquid-impervious backsheet. Furthermore, leg cuffs and adhesive tabs are attached to finalize the hygiene article. The materials and types of pervious topsheet and impervious backsheet and of the leg cuffs and adhesive tabs are known to one skilled in the art and are not particularly restricted. Examples thereof may be found in WO 95/26 209.

The present invention is advantageous in that the esters F, which are useful as crosslinkers, do not have to be purified after they have been formed and particularly in that the (meth)acrylic acid, preferably acrylic acid, does not have to be removed, since it is generally a monomer for forming the hydrogels.

Experimental Part

Parts per million and percentages are by weight, unless otherwise stated.

The example which follows illustrates the process of the present invention.

EXAMPLES

Production of Crude Acrylate Esters Useful as SAP-Crosslinkers

SAP-crosslinkers are prepared in the examples by esterifying polyol or POlol mixtures with acrylic acid by removing water in an azeotropic distillation. The esterification catalyst in the examples is sulfuric acid. The reactants are introduced in the examples as initial charge in methylcyclohexane entrainer together with a stabilizer mixture consisting of hydroquinone monomethyl ether, triphenyl phosphite and hypophosphorous acid. The reaction mixture is then heated to about 98° C. until the azeotropic distillation starts. During the azeotropic distillation, the temperature in the reaction mixture rises. The amount of water removed is determined. The distillation is discontinued once at least the theoretical amount of water has been removed. Subsequently the entrainer is removed in a vacuum distillation. The product is cooled and used as a crosslinker in SAP production.

Conversion and yield of the reaction is not precisely determined because the water removed in the esterification also comprises acrylic acid and acrylic acid is also removed during the vacuum distillation of the entrainer. Similarly, the crude ester still comprises free acrylic acid which is titrated together with the catalyst (acid number). Parts are by weight, unless otherwise stated.

Preparation of Ester

Acid numbers were determined in accordance with DIN EN 3682.

Example 1 Preparation of Alkoxylated Glycol as a Basis for Ester F

a) Trimethylolpropane-30EO-5PO

77 g of trimethylolpropane are placed with 0.5 g of KOH, 45% in water, as an initial charge in an autoclave and dewatered together at 80° C. and reduced pressure (about 20 mbar). 759 g of ethylene oxide are then added at 145 to 155° C. and allowed to react at this temperature under elevated pressure. The reaction has ended when no further change in pressure is observed. The reaction mixture is then stirred for a further 30 min at about 150° C. 167 g of propylene oxide are subsequently added at 120 to 130° C. at elevated pressure over a prolonged period and likewise allowed to react. After purging with inert gas and cooling to 60° C., the catalyst is separated off by addition of sodium pyrophosphate and subsequent filtration.

b) TMP-15-EO is prepared in a similar manner.

c) Tripropylene glycol is a commercially available diol component.

d) Glycerol-3EO is prepared similarly to example 1a).

e) Trimethylolpropane-3EO is prepared similarly to example 1a).

f) Trimethylolpropane-1PO-3EO is prepared similarly to example 1a)—but propylene oxide is added first at 120-130° C. and only then, at 145-155° C., ethylene oxide.

g) Glycerol-30EO-5PO is prepared similarly to example 1a).

h) Butanediol is commercially available.

Example 2 Preparation of Acrylic Ester

a) Trimethylolpropane-30EO-5PO-triacrylate (TMP30EO5POTA)

1427 parts of approximately 30-tuply ethoxylated and 5-tuply propoxylated trimethylolpropane (as per example 1a) is esterified with 216 parts of acrylic acid and 5 parts of sulfuric acid in 345 parts of methylcyclohexane. The assistants added are 2 parts of hydroquinone monomethyl ether and 2 parts of α-tocopherol. 44 parts of water are separated off before the entrainer is removed by vacuum distillation. The product is purified through K300 filter. The acid number is determined. The viscosity is adjusted by addition of 96 parts of acrylic acid. The viscosity of the almost colorless product (iodine number 0-1) is about 330 mPas.

b) TMP-15-EO-triacrylate (TMP15EOTA) is prepared in a similar manner

c) Tripropylene glycol diacrylate (TPGDA) is commercially available as Laromer TPGDA (BASF AG). But it can also be prepared completely analogously to the above examples.

d) Glycerol-3EO-triacrylate (G3EOTA) is prepared in a similar manner.

e) Trimethylolpropane-3EO-triacrylate (TMP3EOTA) is prepared in a similar manner.

f) Trinethylolpropane-1PO-3EO-triacrylate (TMP1PO3EOTA) is prepared in a similar manner.

g) Glycerol-30EO-5PO-triacrylate (G30EO5POTA) is prepared in a similar manner.

h) Butanediol diacrylate (BDDA) is commercially available (BASF AG)

The crosslinker mixtures used in the examples which follow are preparable by simply mixing the ready-prepared acrylate esters in the stated weight ratio. Alternatively, the underlying polyethers can be mixed and conjointly esterified as described above.

Making of Hydrogels

To determine the quality of surface crosslinking, the dried hydrogel can be investigated using the following test methods.

Test Methods

a) Centrifuge Retention Capacity (CRC)

This method measures the free swellability of the hydrogel in a teabag. 0.2000±0.0050 g of dried hydrogel (particle size fraction 106-850 μm) are weighed into a teabag 60×85 mm in size which is subsequently sealed. The teabag is placed for 30 minutes in an excess of 0.9% by weight sodium chloride solution (at least 0.83 l of sodium chloride solution/1 g of polymer powder). The teabag is then centrifuged for 3 minutes at 250 g. The amount of liquid is determined by weighing back the centrifuged teabag.

b) Absorbency Under Load (AUL) (0.7 psi)

The measuring cell for determining AUL 0.7 psi is a Plexiglass cylinder 60 mm in internal diameter and 50 mm in height. Adhesively attached to its underside is a stainless steel sieve bottom having a mesh size of 36 μm. The measuring cell further includes a plastic plate having a diameter of 59 mm and a weight which can be placed in the measuring cell together with the plastic plate. The plastic plate and the weight together weigh 1345 g. AUL 0.7 psi is determined by determining the weight of the empty Plexiglass cylinder and of the plastic plate and recording it as W₀. 0.900±0.005 g of hydrogel-forming polymer (particle size distribution 150-800 μm) is then weighed into the Plexiglass cylinder and distributed very uniformly over the stainless steel sieve bottom. The plastic plate is then carefully placed in the Plexiglass cylinder, the entire unit is weighed and the weight is recorded as W_(a). The weight is then placed on the plastic plate in the Plexiglass cylinder. A ceramic filter plate 120 mm in diameter, 10 mm in height and 0 in porosity (Duran, from Schott) is then placed in the middle of a Petri dish 200 mm in diameter and 30 mm in height and sufficient 0.9% by weight sodium chloride solution is introduced for the surface of the liquid to be level with the filter plate surface without the surface of the filter plate being wetted. A round filter paper 90 mm in diameter and <20 μm in pore size (S&S 589 Schwarzband from Schleicher & Schüll) is subsequently placed on the ceramic plate. The Plexiglass cylinder comprising hydrogel-forming polymer is then placed with plastic plate and weight on top of the filter paper and left there for 60 minutes. At the end of this period, the complete unit is removed from the filter paper and the Petri dish and subsequently the weight is removed from the Plexiglass cylinder. The Plexiglass cylinder containing swollen hydrogel is weighed together with the plastic plate and the weight recorded as W_(b).

AUL was calculated by the following equation: AUL 0.7 psi [g/g]=[W _(b) −W _(a) ]/[W _(a) −W ₀]

AUL 0.5 psi is measured in similar fashion at a lower pressure.

c) The 16 h extractables value is determined similarly to the description in EP-A1 811 636 at page 13 line 1 to line 19.

Example 3 Preparation of Superabsorbent Using the Acrylic Ester from Example 2 and Mixtures Thereof Example A (Comparative Example)

A Lödige VT 5R-MK plowshare kneader (5 l volume) is charged with 180 g of deionized water, 220 g of acrylic acid, 2201 g of a 37.3% by weight sodium acrylate solution (100 mol % neutralized) and also 5.1 g (=0.60% by weight based on acrylic acid monomer) of the crosslinker trimethylolpropane-15EOtriacrylate. This initial charge is inertized by having nitrogen bubble through it for 20 minutes. Dilute aqueous solutions of 2.112 g of sodium persulfate, 0.045 g of ascorbic acid and also 0.126 g of hydrogen peroxide are added to start the reaction at about 23° C. After the reaction has started, the temperature of the heating jacket is controlled to the reaction temperature in the reactor. The polymerization in the kneader is carried out with stirring and thorough mixing. The crumbly gel eventually obtained is then dried in a circulating air drying cabinet at 160° C. for about 3 h. This is followed by grinding and classifying to 300-850 micrometers. The gel obtained is finally characterized.

Further examples which follow are prepared similarly to example 3A: TABLE 1 Amount used Amount Ex. based on acrylic used No. Crosslinker type acid monomer in g A Trimethylolpropane-15 EO-triacrylate 0.60% by weight 5.1 g B Trimethylolpropane-15 EO-triacrylate 2.00% by weight 17.0 g  C TMP-30 EO-5 PO-triacrylate 0.60% by weight 5.1 g (69.3% by weight) Laromer TPGDA (30.7% by weight) D Glycerol-3 EO-triacrylate (25.2% by weight) 0.60% by weight 5.1 g Trimethylolpropane-30 EO-5 PO-triacrylate (74.8% by weight) E Trimethylolpropane-3 EO-triacrylate 2.00% by weight 17.0 g  (28.8% by weight) Trimethylolpropane-30 EO-5 PO-triacrylate (71.2% by weight) F Trimethylolpropane-1 PO-3 EO-triacrylate 0.60% by weight 5.1 g (34.0% by weight) Trimethylolpropane-30 EO-5 PO-triacrylate (66.0% by weight) G Butanediol diacrylate (18.3% by weight) 0.60% by weight 5.1 g Trimethylolpropane-30 EO-5 PO-triacrylate (81.7% by weight) H Glycerol-30 EO-5 PO-triacrylate (69.8% by weight) 2.00% by weight 17.0 g  Laromer TPGDA (30.2% by weight) I TMP-30 EO-5 PO-triacrylate (7.7% by weight) 0.30% by weight 2.6 g Laromer TPGDA (92.3% by weight)

The properties of these hydrogels are summarized in tab. 2 CRC Extractables 16 h AUL 0.3 psi Ex. [g/g] [% by weight] [g/g] A TMP15EOTA 0.6 35.8 9.5% 16.6 B TMP15EOTA 2.0 26.4 5.1% 25.0 C TMP30EO5POTA 36.0 10.1% 16.7 TPGDA 0.6 D TMP30EO5POTA 36.7 8.6% 17.0 G3EOTA 0.6 E TMP30EO5POTA 27.0 4.3% 25.3 TMP3EOTA 2.0 F TMP30EO5POTA 37.1 8.8% 15.9 TMP1PO3EOTA 0.6 G TMP30EO5POTA 36.6 8.6% 16.2 BDDA 0.6 H G30EO5POTA 27.6 4.5% 24.7 TPGDA 2.0 I TMP30EO5POTA 35.0 10.0% 18.0 TPGDA 0.3

As can be seen from tab. 2, the properties of the crosslinker of the comparative examples can be obtained in a similar manner using a multiplicity of other crosslinkers in amounts which have been calculated according to the present invention.

In addition, example 3I discloses how the crosslinker strength can be adjusted according to the present invention, by varying the mixture fractions. This can be taken advantage of in industry by keeping the total amount of crosslinker added constant by suitably mixing the components and feedback controlling the strength of crosslinking through the component ratio only. This represents an industrial advantage because in this way the mixing of crosslinker into the monomer solution can optimally be adjusted on a large industrial scale using constant flow streams.

Postcrosslinking:

The dry normal base polymer powder is sprayed homogeneously (while stirring) with a solution of 0.12% by weight of N-hydroxyethyl-2-oxazolidinone, 3.35% by weight of water and 1.65% by weight of 1,2-propanediol, all percentages being based on polymer used.

The batch size is in each case 1.2 kg, and spraying is through a two-material nozzle by atomizing the solution with nitrogen. A plowshare mixer from Lödige is used which has a 5 l working volume.

The moist powder is then heat treated in a drying cabinet at 180° C. for 60 min. It is then sieved once more at 850 micrometers in order that agglomerates may be removed.

Only about 100 g of moist polymer are needed for drying.

The properties of this postcrosslinked polymer are determined. Ex. Base polymer CRC AUL 0.7 psi A 3A (comp. ex.) 29.9 24.0 A1 3B (comp. ex.) 24.5 21.9 A2 3C 30.2 24.5 B 3E 25.0 22.4

In a further experiment, a fraction having a particle size 300 to 850 μm was prepared and surface postcrosslinked with ethylene glycol diglycidyl ether (0.10% by weight based on polymer). Apparatus and amounts of all assistants used are exactly the same as described above. However, the moist polymer was dried in the circulation air cabinet at only 150° C. for 1 hour.

Various performance data are raised in order that the various crosslinkers may be compared under constant conditions. % by weight of crosslinker AUL AUL based on CRC 0.3 0.7 Extractables Crosslinker acrylic acid g/g psi g/g psi g/g 16 h TMP-15EO- 1.0 30.0 29.0 23.0 9.0 triacrylate 2.0 25.3 26.4 22.6 5.1 3.0 23.7 25.3 21.6 3.2 TPGDA 1.0 33.8 32.3 24.3 17.3 2.0 26.6 27.8 23.8 5.0 3.0 24.8 26.3 22.5 4.4 TMP-30EO- 1.0 32.9 31.8 25.0 13.7 5PO-triacrylate 2.0 32.2 30.0 22.8 11.7 3.0 29.3 29.1 23.2 8.0 70% by weight 0.25 31.6 30.5 23.6 11.1 of TMP-30EO- 0.35 32.1 30.9 23.8 11.4 5PO-triacrylate 0.50 31.2 30.8 24.5 9.1 30% by weight 0.70 29.9 30.2 24.5 6.9 of TPGDA 1.00 28.7 29.5 23.9 5.9 1.50 27.4 28.1 23.6 4.8 2.00 26.6 27.7 23.4 4.4

It is evident from the preceding table that the inventive mixture of two crosslinkers is superior to the use of the individual crosslinker components and also superior to a single similar crosslinker having a similar WFR.

For instance, the extractables value of about 9 is achieved with just 0.5% by weight of inventive crosslinker combination compared with more than 1.0% or even more than 2.0% by weight of crosslinker fraction of the individual components and about 1.0% by weight fraction of the similar individual crosslinker. The CRC and AUL values of the crosslinker combination are likewise superior to those of similar individual crosslinker for the same extractables value.

A further experiment was carried out to prepare 6 different mixtures in order that the invention may be demonstrated. Hydrogel-forming polymers are prepared therewith as described in example A, except that the pure crosslinker used there is replaced by an equal amount of one of the mixtures in each case. The postcrosslinking was then carried out as described above with ethylene glycol diglycidyl ether using the 300-850 μm fraction. % by Use level weight Mole based on in fraction acrylic Extractables Mixture Crosslinker mixture Functionality M_(w) M_(w)/F (α) acid CRC AUL 0.7 psi 16 h 1 Trimethylolpropane 21.4% 3 338 113 0.61 0.6 wt % 30.5 24.7 9.0% trimethacrylate TMP-30 EO-5 PO-triacrylates 78.6% 3 1906 635 0.39 2 Glycerol diacrylate 18.6% 2 200 100 0.59 0.6 wt % 31.0 25 8.9% TMP-30 EO-5 PO triacrylate 81.4% 3 1906 635 0.41 3 Polyethylene glycol 300 47.0% 2 408 204 0.73 0.6 wt % 30.3 24.3 8.2% diacrylate TMP-30 EO-5 PO triacrylate 53.0% 3 1906 635 0.27 4 Glycerol-3 PO-triacrylate 28.8% 3 428 143 0.64 0.6 wt % 30.1 25.2 7.9% TMP-30 EO-5 PO-triacrylate 71.2% 3 1906 635 0.36 5 Glycerol-3 EO-triacrylate 26.6% 3 386 129 0.66 0.6 wt % 30.8 25.6 9.7% TMP-40 EO-TA 73.4% 3 2056 685 0.34 6 Glycerol-3 EO-triacrylate 26.2% 3 386 129 0.65 0.6 wt % 30.1 24.9 8.0% Glycerol-40 EO-triacrylate 73.8% 3 2014 671 0.35 

1. A mixture of at least two compounds each having at least two double bonds, said mixture having a WFR from 240 to 600 g/mol of double bond and at least two of said compounds each comprising at least two (meth)acrylic esters as double bond component, WFR being given by: ${\sum\limits_{i = 1}^{n}{{\alpha_{i} \times {MW}_{i}}/Z_{i}}} = {{WFR}\quad{where}}$ ${\sum\limits_{i = 1}^{n}\alpha_{i}} = 1$ α_(i) is equal to the molar fraction of compound (i) in said mixture, n is equal to the number of compounds in said mixture and n is >2, Z_(i) is equal to the number of double bonds in said compound (i), MW_(i) is equal to the molecular weight of said compound (i).
 2. The mixture of claim 1 having a WFR between 240 and 400 g/mol of double bond.
 3. The mixture of claim 1 wherein n is 2, 3, or
 4. 4. The mixture of claim 1 wherein the MW/Z ratios of two compounds differ by at least 50 g/mol of double bond.
 5. The mixture of claim 1 wherein one compound has an MW/Z ratio of below 400 g/mol of double bond.
 6. The mixture of claim 1 wherein one compound has an MW/Z ratio of above 400 g/mol of double bond and below 10 000 g/mol of double bond.
 7. The mixture of claim 1 wherein Z of at least one compound is between 2 and
 6. 8. The mixture of claim 1 wherein said compounds are esters F_(i) which are obtainable by esterification of polyalcohols A_(i) with (meth)acrylic acid and each polyalcohol A_(i) has Z_(i) hydroxyl functions and from 2 to 50 carbon atoms.
 9. The mixture of claim 1 wherein one compound is represented by one of the following formulae:

wherein AO is independently at each instance —O—CHR7-CHR8- or —CHR7-CHR8-O— wherein R7 and R8 are independently H, linear or branched C1-C8-alkyl, R5 and R6 are independently H, linear or branched C1-C8-alkyl, n is 1, 2, or 3, p1 is 0, 1, or 2, p2 is 0, 1, or 2, p3 is 0, 1, or 2, p4 is 0, 1, or 2, and R1, R2, R3, R4 are independently H or CH3.
 10. The mixture of claim 1 wherein one compound is represented by one of the following formulae:

wherein AO is independently at each instance —O—CHR7-CHR8- or —CHR7-CHR8-O—, wherein R7 and R8 are independently H, linear or branched C1-C8-alkyl, R5 and R6 are independently H, linear or branched C1-C8-alkyl, n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20, p1 is 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20, p2 is 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20, p3 is 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20, p4 is 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20, and R1, R2, R3, R4 are independently H or CH3.
 11. The mixture of claim 9 wherein AO is independently at each instance EO or PO, wherein EO is O—CH2-CH2-, PO is independently O—CH2-CH(CH3)- or O—CH(CH3)-CH2- R5 and R6 are independently H or CH3.
 12. A process for preparing an ester mixture of said esters F_(i) of claim 8 by starting from an alcohol mixture of polyalcohols A_(i), comprising the steps of a) reacting said polyalcohols A_(i) with (meth)acrylic acid in the presence of at least one esterification catalyst C, at least one polymerization inhibitor D, and optionally a water-azeotroping solvent E to form an ester mixture of said esters F_(i), b) optionally removing from the reaction mixture some or all of the water formed in a), during and/or after a), f) optionally neutralizing said reaction mixture, h) when a solvent E is used, optionally removing the solvent E by distillation, and/or i) stripping the reaction mixture with a gas which is inert under the reaction conditions.
 13. The process for preparing an ester mixture of said esters F_(i) according to claim 12 wherein a molar excess of (meth)acrylic acid over said polyalcohols A_(i) is at least 5*Z_(i) mol %, and the optionally neutralized (meth)acrylic acid present in said reaction mixture after the last process step substantially remains in said reaction mixture.
 14. The process for preparing an ester mixture according to claim 12 wherein the (meth)acrylic acid is not more than 75% by weight removed from said reaction mixture obtained after said last step, which reaction mixture comprises the ester mixture.
 15. The process for preparing an ester mixture according to claim 12 wherein said reaction mixture obtained after said last process step, which comprises the ester mixture, has a DIN EN 3682 acid number of at least 25 mg KOH/g.
 16. The process for preparing an ester mixture according to claim 12 wherein said reaction mixture obtained after said last process step, which comprises the ester mixture, has a (meth)acrylic acid content of at least 0.5% by weight.
 17. The process for preparing an ester mixture according to claim 12 wherein the molar ratio of (meth)acrylic acid to alcohol mixture A_(i) in reaction step a) is at least 5*Z_(i):1.
 18. A process for preparing a crosslinked hydrogel, comprising the steps of k) polymerizing an ester mixture of claim 1 with (meth)acrylic acid, optionally with an additional monoethylenically unsaturated compound N, and optionally at least one further copolymerizable hydrophilic monomer M, in the presence of at least one free-radical initiator K and optionally at least one grafting base L, l) optionally postcrosslinking the reaction mixture obtained from k), m) drying the reaction mixture obtained from k) or l), and n) optionally grinding and/or sieving the reaction mixture obtained from k), l), or m).
 19. A process for preparing a crosslinked hydrogel, comprising steps a) to i) according to claim 12 and additionally k) polymerizing the reaction mixture from one of steps a) to i) of claim 12, if performed, optionally with an additional monoethylenically unsaturated compound N and optionally at least one further copolymerizable hydrophilic monomer M, in the presence of at least one free-radical initiator K and optionally at least one grafting base L, l) optionally postcrosslinking the reaction mixture obtained from k), m) drying the reaction mixture obtained from k) or l), and n) optionally grinding and/or sieving the reaction mixture obtained from k), 1), or m).
 20. A polymer prepared according to the process of claim
 18. 21. A crosslinked hydrogel comprising at least one hydrophilic monomer M is copolymerized from crosslinked with an ester mixture having a WFR from 200 to 600 g/mol of double bond according to claim
 1. 22. (canceled)
 23. (canceled)
 24. A composition comprising from 0.1% to 40% by weight of at least one ester mixture of of claim 1 and (meth)acrylic acid, 0.5-99.9% by weight of at least one hydrophilic monomer M, 0-10% by weight of at least one esterification catalyst C, 0-5% by weight of at least one polymerization inhibitor D, and 0-10% by weight of at least one solvent E, with a proviso that the sum total is 100% by weight.
 25. The composition of claim 24, further comprising a diluent G.
 26. A crosslinked hydrogel prepared from a composition of claim 24 and additionally optionally postcrosslinked.
 27. (canceled)
 28. The crosslinked hydrogel of claim 21 which has a residual crosslinker content of less than 10 ppm.
 29. (canceled)
 30. (canceled)
 31. The mixture of claim 2 having a WFR between 250 and 350 g/mol of double bond.
 32. The mixture of claim 3 wherein n is
 2. 33. The mixture of claim 4 wherein the MW/Z ratios of two compounds differ by at least 100 g/mol of double bond.
 34. The mixture of claim 5 wherein one compound has an MW/Z ratio of below 300 g/mol of double bond.
 35. The mixture of claim 6 wherein one compound has an MW/Z ratio of above 600 g/mol of double bond and below 1000 g/mol of double bond.
 36. The mixture of claim 10 wherein AO is independently at each instance EO or PO, wherein EO is O—CH2-CH2-, PO is independently O—CH2-CH(CH3)- or O—CH(CH3)-CH2- R5 and R6 are independently H or CH3.
 37. The method of claim 13 wherein said mixture of esters F_(i) have a WFR from 200 to 600 g/mol of double bond.
 38. A polymer prepared according to the process of claim
 19. 39. An article comprising a polymer prepared according to the method of claim
 12. 40. The article of claim 39 selected from the group consisting of a hygiene article, a packaging material, and a nonwoven.
 41. A method of absorbing aqueous fluids comprising contacting the fluid with a hydrogel-forming polymer crosslinked using an ester mixture of claim
 1. 42. The method of claim 41 wherein each ester is present in an amount of less than 2% by weight, based on the total amount of monomer. 